The point in the question is neither hundred percent acceptable nor is it hundred percent refutable, but it depends squarely on the type of the sin and the person who commits it. There are some sins like shirk (polytheism) which is not forgivable at all. Similarly, violating or not fulfilling people's rights is a sin which is not forgivable through others' prayers and invocation. In fact, it is forgiven only through compensation and satisfying those whose rights have not been fulfilled.
Sometimes a sinner commits a sin due to ignorance in which case he is likely to be forgiven after he turns to Allah with regret and repentance. Sometimes, he commits a sin out of mere rebellion and disobedience in which case he needs to compensate for his mistake and redeem himself. Compensation for the bad effect of the sin can be made by the sinner himself or by other individuals.
Of course, man does not deserve to get more than what he earns through endeavor and efforts. However, it does not mean that Gods' bounties will cease to reach him through divine grace and favor. Being entitled is one thing and divine grace is another.
Moreover, others' act of kindness, favor, prayers and intercession do not remain unaccounted for as they somehow originate in one's own individual effort. For example, intercession needs endeavor and creating a spiritual relationship with the intercessor. Intercession requires eligibility and growth ability in the same way as a plant cannot grow to a perfect level until it receives water, earth, light and have all the potentials and capacity to grow with the help of these elements.
To clarify the answer to the question, we must make mention of the following points:
1. Every sin has two aspects one of which relates to the act of defiance or disobedience (an oppression committed against God) and another relates to the sinner cum other individuals of society (oppression committed against people and the sinner).
2. Keeping in view the various circumstances in which a sin is committed, the sins and the sinners are not at the same level and they do not get the same treatment.
For example, the Quran, addressing the Holy Prophet's wives, says: "O wives of the prophet! whoever of you commits an open indecency, the punishment shall be increased to her doubly; and this is easy to Allah."
3. In another categorization, the sins are divided into two kinds – the rights of God and the rights of people – and it has been narrated that: "God never forgives [the sinner who does not fulfill people's rights except for when the owner of the right (he whose right has been violated) exempts him." Thus, the sins that relate to human rights are never forgivable through prayers and repentance of the sinner or other people irrespective of whether forgiveness is sought during the sinner's lifetime or after his death. Unless the people whose rights have been violated give their consent and exempt the violator of their rights he remains liable to divine punishment. For example, if a person has usurped another person's property or caused it to perish, God will not forgive him unless he whose right has been violated exempts the sinner or forgoes his right.
4. The sins are divided into minor and major ones. A major sin is a sin, which according to Islam, is so important that it has not only been strictly forbidden but God, the Almighty, has also promised to punish the sinner. These sins include fornication, murder of an innocent person, receiving usury etc. as stated in the narrations. As for a minor sin, it is a sin that has been forbidden only.
The Holy Quran has promised to forgive the minor sin. It says:
"If you shun the great sins which you are forbidden, We will do away with your small sins and cause you to enter an honorable place of entering."
According to the holy verse, if a person abstains from major sins, his minor sins will also be forgiven. However, it should be noted that under certain circumstances, the minor sins change into major ones and the sinner will be subjected to punishment in Hell. That is, most of the minor sins change into major ones whose perpetrator God has promised to punish.
5. The sinners are divided into two groups:
a) Those who commit sins on account of ignorance and lack of knowledge in which case he is forgiven after he turns to Allah with regret and repentance. God, the Glorified, has promised to forgive those who repent and make up for their sin. The divine promise is fulfilled exclusively in regard to them.
b) Those sinners who commit sins knowingly and do not seek repentance will not be forgiven. There is no doubt that divine forgiveness will not include them.
It is clear from the above that divine forgiveness and any change in the situation of the sinners are possible only with those sins which God has promised to forgive after the causes for forgiveness has been made available.
As for how a person who has been deviated and committing sins throughout his life is forgiven with others' invocation and good deeds, we should say that it is not true with all deviated and sinful people; the situation of all the sinners will not change with others' prayers and invocation. In fact, there are some sins which God has promised to forgive even with others' prayers and good deeds because God says, "the good deeds wipe out (the effects) of the bad deeds."
It does not make a difference whether the sinner himself does good deeds to wipe out the effects his sins or someone else does good deeds and dedicates its reward to the sinful person. Good deeds, as indicated in the traditions, wipe out the sins and their effects.
We have many traditions which says that when a person passes away from this world, and others (like his sons or other people) do good deeds such as prayers, fasting, Hajj, Sadaqa, freeing a slave etc. and dedicate the reward of these actions to the deceased person, the rewards of these actions will reach him.
Certainly, if the reward of the good deeds done by another person and dedicated to the sinner is plentiful enough to wipe out the effects of the sinner's sin, it will make up for his sin also. Divine justice requires the sins committed by the sinner to be wiped out by the good deeds that serve as worthy compensation. God, the Exalted, has opened this way [to the servants of God] on the tongue of His Prophet (pbuh).
In this world also, if a person commits an offence and harms another person, and if a third individual reimburses him for the damage, he will surely be satisfied and if he is not satisfied, others' will reproach him. Therefore, one should not wonder and question as to how the sin of a sinner who has died is compensated by other people's good deeds.
Definitely, if a person has been committing so many sins that even if all the good-doing people pray for him and dedicate the rewards of their deeds to him, they will not make up for his numerous sins, in this case, his sins will never be forgiven with others' good deeds; rather his sins will be reduced and that God will consider some concession for him.
It should be noted that since God, the Glorified, is Just and Wise and He does not commit any oppression against any of His servants and does not waste their rewards, it is likely that God may forgive those whom we consider to be sinful and entitled to punishment (because we are not fully aware of their actions and deeds), but God has knowledge of everything and He knows things which we are unaware of. If he forgives someone, it should be because of the good deeds which we have no knowledge of.
It should also be noted that since invocation and seeking forgiveness for others are often because of the positive aspects and good deeds which they have done, otherwise, if there is someone who does not have a single positive point in his life and that all his life has been full of sins, no one including his relatives will pray sincerely for his forgiveness. And if his relatives themselves are bad people, their prayers will not be answered and if they are good people, they do not remember any good thing about him so as to pray beseechingly for him.
When it comes to the consistency between verse 21 of Chapter Tur which says "And (as for) those who believe and their offspring follow them in faith, We will unite with them their offspring and We will not deprive them (of the fruit) of aught of their works" whereas the offspring has not made any endeavor and also the prophetic tradition according to which whoever performs a good deed, the fruit of their good deeds reach his children and the verse which says "there is not anything for man except what he achieves through endeavor", they are reconcilable. According to the Quran, man does not deserve to get more than what he earns through endeavor and efforts. However, it does not mean that Gods' bounties will cease to reach him through divine grace and favor. Being entitled is one thing and divine grace is another. If a person does a good deed, God bestows him rewards ten folds or hundred folds of the good deed.
Besides this, intercession is not something easy to achieve. It also needs some endeavor and spiritual communion with the intercessor. Also as to the joining of the offspring of those who dwell in Paradise, the Quran says that they will join them only if they follow them in faith.
In regards to intercessions, Allamah Muhammad Hussein Tabatabai says in his Tafsir al-Mizan: "In other words, when he wants to get a reward without doing his task, or to save himself from punishment without performing his duty, then he looks for someone to intercede on his behalf. But intercession is effective only if the person for whom one intercedes is otherwise qualified to get the reward and has already established a relationship with the authority. If an ignorant person desires appointment to a prestigious academic post, no intercession can do him any good; nor can it avail in case of a rebellions traitor who shows no remorse for his misdeeds and does not submit to the lawful authorities. It clearly shows that intercession works as a supplement to the cause; it is not an independent cause."
 - Tafsir Nomouneh, vol.6, pg.415.
 - Al-Ahzab: 30
 - Al-Kafi, vol.2, pg.428
 - Tafsir Nomouneh, vol.14, pg.406.
 - Al-Kafi, vol.2, pg.433
 - "الکبائر التی اوجب الله عزوجل علیه النار" The greater sins are those for which God, the Glorified, has made Hell fire obligatory. Al-Kafi, vol.2, pg.276.
 - "ان تجتنبوا کبائر ما تنهون عنه نکفر عنکم سئیاتکم و ندخلکم مدخلاً کریماً" An-Nisa:31
 - The conditions under which the minor sins change into major ones are the following:
1. If the minor sins are repeated: Imam Sadiq (a.s.) said: “There is no minor sin with persistence in it” meaning, if a person keeps up committing minor sins, they will change into major ones. [al-Kafi, vol.2, pg.88)
2. If the sinner considers minor sins as petty and insignificant, they will change into major ones. Imam Ali (a.s.) said: “The biggest sin is to consider a sin, which one commits, as small.” [Wasail al-Shia, vol.15, pg.312, chapter 43]
3. If the sinner commits the sin to disobey God’s command out of arrogance and rebellion, the sin changes into a major one. The Holy Quran says, “Those who have rebelled and preferred the worldly life, hell will be their dwelling.” [Quran: 37-39]
4. If a minor sin is committed by a prominent person of high social position, it will change into minor ones. Speaking about the Prophet’s wives, the Quran says: “Wives of the Prophet, if anyone among you commits indecency, her torment will be double. This is not at all difficult for God. [Quran: 30]
5. If the sinner is happy doing the sin, the sin will change into a major one. The Holy Prophet (pbuh) said: “Whoever commits a sin while laughing, he will enter fire crying.” Wasail al-Shia, vol.15, pg.305, chapter 40.
6. If the sinner considers the delay in the punishment as a sign of God’s being pleased with him. The Quran quotes some of the arrogant sinners who said: "Why has God not punished us for what we say (if he is a true Prophet)?" The heat of hell is a sufficient torment for them. This is the most terrible fate.” Al-Mujadilah: 8. Read Tafsir Nomouneh, vol.3, pg.359 onward.
 - Of course, it should be noted that divine punishment is in no way inconsistent with seeking intercession. Thus, those who have committed major sins can still seek intercession. In regards to where and who intercession applies to, Allamah Tabatabai, the great exponent of the Quran says: “It has been described that intercession takes place in two spheres: in creation and in legislation.
The intercession in creation is related to every cause in this world of the cause and effect.
As for the intercession in matters of legislation and Judgment, some of them wipe out every sin and its punishment, right from polytheism to the smallest one. For example, repentance done, and true faith acquired, before the Day of Resurrection. Some wipe out effects of some particular sins, like some specified good deeds. As for the issue under discussion, i.e., the intercession of the prophets and other believers on the Day of Judgment, we have already explained that it shall avail those believers who might have committed big sins, but whose faith Allah is pleased with.” Tafsir al-Mizan: vol.1, pg.264.
 - The Quran says: “God will only accept the repentance of those who commit evil in ignorance, if they repent immediately. God is All-knowing and All-wise” [al-Nisa: 17]
 - The exponents and commentators of the Quran have with reliance on Quranic verses and traditions enumerated the causes for forgiveness as such:
1. Repentance and regretting over the sins he has committed in the past and at the same time deciding not to commit them. The Quran says: " ""و الذی یقبل التوبة عن عباده و یعفو عن السیئات... He is the One that accepts repentance from His Servants and forgives sins: and He knows all that ye do. [Ash-Shura:25]
2. Doing good deeds which cause the bad deeds to be wiped out. The Quran says, "إن الحسنات یذهبن السیئات" . Surely good deeds take away evil deeds. [Hud:114]
3. Intercession: Intercession is the Prophet's invocation or that of one of the Imams. Intercession of the intercessors (e.g., the prophets, the friends of Allah and those who are nearer to Him) is one of those causes, and certainly no rashness or injustice is entailed therein. See Bohuth Qur'aniyah by Ja'far Subhani, pg. 117-118.
Intercession, as explained above, is a confirmed reality not in every case but in approved ones. The Qur'an and the traditions do not prove more than this. A little meditation on the meaning of intercession is enough to lead to this conclusion. Intercession is mediation in causality and effectiveness. Obviously causality cannot be limitless and unconditional. No cause can be a cause of every effect, nor can an effect be governed by every cause - otherwise it would render the system of cause and effect null and void. Those who do not believe in intercession have fallen in this very trap - they thought that we affirm the intercession in its totality without any condition or limit. All their objections emanates from this very misunderstanding.
4. If a person stays away from major sins, his minor sins will also be forgiven. See Chapter al-Nisa verse 31.
5. Divine forgiveness includes those who deserve to be forgiven. It depends on God's will and is applicable definitely only to those who have somehow in practice proved to be deserving it.
It becomes clear here as to why God, the Glorified, has said that shirk (polytheism) is not forgiven. [Allah does not forgive that any thing should be associated with Him, and forgives what is besides that to whomsoever He pleases (4:48).] Polytheism is not forgiven because the polytheist has cut off his relationship with God and has committed an action that is opposed to the principles of all religions.
Perhaps, among the causes enumerated above, repentance is the strongest of all causes. God, the Exalted, calls on all His servants to seek repentance. See verse 8 of al-Tahrim, 60 of Hud and 31 of al-Noor in which God says that He "loves those who turn to Him constantly". Allah says: "Say: ...O my servants! who have acted extravagantly against their own souls, do not despair of the mercy of Allah; surely Allah forgives the faults altogether; surely He is the Forgiving, the Merciful. And return to your Lord..".(Al-Zumar:53 - 54) He also says: "Unless he repents, believes, and works righteous deeds, for Allah will change the evil of such persons into good, and Allah is Oft-Forgiving, Most Merciful." [al-Furqan: 70.]
It should be noted that repentance should take place in a propitious time and under specific conditions so that it may be effective. Some of the conditions for repentance are the following: 1- The sinner should be regretful and penitent. 2. He should compensate his sinful actions by doing good deeds. (If he has wasted other people's rights, he should fulfill their rights, make them up and satisfy them.) 3. He should decide not to return to the sin again. 4. He should have repented at the right time not at a time when he feels death approaching him.
 - Hasanāt
 - Surely good deeds take away evil deeds. [Hud:114]
 - Wasail al-Shia, vol.2, pg.445, hadiths 2603, 2605, 2606; vol.3, pg.200, vol.6, pg.219; vol.11, pg.204.
 - See: Mizan al-Hikmah, Tawbah sect [letter T].
 - There is no doubt that polytheism is treated in a special manner. In regards to polytheism, God, the Glorified says, " It is not (fit) for the Prophet and those who believe that they should ask forgiveness for the polytheists, even though they should be near relatives, after it has become clear to them that they are inmates of the flaming fire." Then when it comes to Ibrahim's seeking forgiveness for Azar, it says "And Abraham prayed for his father's forgiveness only because of a promise he had made to him." Tafsir Nomouneh, vol.5, pg.304.
 - For further information about the meaning and conditions of intercession, see Tafsir Nomouneh, vo.13, pg.304; vol.9, pg.90; vo.1, pg.228; vol.1, pg.229; vol. 1, pg.235 and (Farsi translation of) Tafsir al-Mizan, vol.1, pg.238-265.
 - Tafsir Nomouneh, vol.22, pg.555 – 556.
 - Translation of Tafsir al-Mizan, vol.1, pg.240.