Stats should play some role - but they shouldn't be the dominant factor, imo.
Just because a player has one good year doesn't mean he's going to produce even better years as time goes on.
Even great players don't do that. They just keep playing at a high level consistently. Upping a players' ratings after good years makes it more likely that they'll keep putting up even bigger numbers (if they did very well with average ratings, they'll be OMGLOLWUT good with great ratings).
Players don't get better at football by playing football well. They play football better because they improved their skills.
Then there comes a point where the player doesn't get better or worse, but plays at a certain range of performance. For great players this is a higher level for a longer time. For weaker players, the peaks are likely smaller and shorter duration.
The game needs a better way of improving player skills, imo. Then the stats will come as a result of improving (or declining) skills. That's where you have to utilize the player differently so you get the most out of his skills in order to improve/maintain his performance for the team.