If there are guys that improve on regular basis. There must be guys who decline on regular basis. Decline does not start at 33 years old in real world aswell. We have seen guys being at All-Star level decline in 2 seasons to a few minutes off the bench men (with no injury).
Looking at BB, this improving and declining on random, would just ruin the game. Too much random, is sometimes just too much. We currently have a good system, with the great addition of cross-training. I think it's logical to assume, that everyone in your team get's training. That training keeps them at their current level and keeps them from declining. The guys you give individual training to will improve, if they get to use those newly acquaired skills in games, enough minutes per week.