Using heaven to argue for hell

Against the idea that hell is eternal, people often argue that it is unjust to punish someone for an infinite amount of time for a finite number of sins. This is because each sin is only finitely wrong and hence only deserves a finite amount of punishment. Since one can only commit a finite number of sins, there is only a finite amount of wrong they can commit. Hence, they would only deserve a finite amount of suffering, not an infinite amount. So, punishing someone in hell for an infinite amount of time is wrong.

Sometimes to counter this, Christians argue that the argument can go against sending people to heaven for eternity as well. Each person who goes to heaven only commits a finite number of good acts in their life and hence is only entitled to a finite amount of reward. Any more reward than what they deserve would be unjust. Therefore, it would be unjust for God to send people to heaven for all eternity.

However, since it is not unjust for God to send people to heaven, then the argument has been reduced to absurdity so it must be unsound.

This doesn't work however, because there is a necessary disanalogy between punishing people more than they deserve and rewarding people more than they deserve. If we give people good things, even give them good things that they don't deserve, it is not wrong to do so. It actually may be supererogatory in some circumstances. For example, when you give money to the poor, perhaps the poor don't "deserve" your money. However, that doesn't show that it's wrong or unjust to give to the poor. It is still a good thing to do.

However, it is wrong and unjust to punish someone more than they deserve. If I deserve n days of punishment, punishing me for n+1 days would be wrong. The same with any more amount of punishment than what I deserve. So, the argument can't be reduced to absurdity because the statement " Any more reward than what they deserve would be unjust" is false. There is a disanalogy between punishing people more than they deserve and rewarding people more than they deserve.