The mistaken assumption here is that a finite number of sins cannot have a (potentially) infinite number of harmful effects. If I wrong someone, I by virtue of that act worsen their state; I render them less happy, less well off, than they would otherwise be. Because of this, they therefore are not able to communicate quite as much goodness, or happiness, or well-being to others as they would have been able to had I not lessened their overall state through my wrongful act. And, of course, those folks would then not be able to affect others as positively as they would have been able to otherwise, and so on ad infinitum. Since even a single sin has never-ending effects (since the stream of events effects people who will enter eternity different, even if in a small way, from what they would have been otherwise), it follows that a never-ending consequence for such acts is justified.