Bingo! The only reason a team should consent to renegotiating a player's salary is to lock-up a young, IMPROVING player -- not one who has already peaked. The one exception might be the quarterback position. Many QB's can be quite productive long after they lose that youthful zip on the ball.
Given how frequently players try to reneg on their contracts, I am surprised that teams haven't just started negotiating performance-based contracts in order to avoid this issue. I suppose the reason why is that the players don't want to be judged on performance, particularly when they know they can always whine and get out of their contracts if they want a better one.