Because older players getting worse is a problem with the game?
If franchise is supposed to be simulating the growth and decline of NFL players? Why would an older HB get better?
Stay the same/decline slowly? Possibly. Ray Lewis is still doing his thing, but I wouldn't say he's getting BETTER. If anything, he's slowing down physically, but his experience and other "intangibles" allow him to still be very productive as well as the fact he's declining from a high point of ability, so it will likely take longer before he's unproductive.
That's where the problem in Madden's progression is, imo, "intangibles"/techniques don't overcome physical declines (or raise even if physical abilities drop because things don't rise and fall independently). Not that there's no "reward the user" gains for players.