That is were you are wrong. In madden everything is definitive to what his ratings are. No more, no less.
A player's stats in madden are the pre-determined actuality that everyone thinks they will be able to perform up to that season. Potential is simply saying "We know that not all players were created equal and therefore we need some system to stop any player from becoming great"
Without Potential myself, you, your 12 grade math teacher in madden could turn into the next HOF. How realistic is that? It is considered a fair compromise to "shaft" real life players potential with assumptions about how they may perform to create a fantasy world that reflects the real world as much as possible.