That ages too. Drastically.
If you want a narrative for that, try this: As a player ages, his bat slows down. This is noticed by pitchers/catchers across the league. They start challenging the hitter on the inner 3rd with velo. To compensate, the hitter cheats more and starts his swing before he's used to. Because of that, the time his former abilities afforded to recognize and take borderline pitches degrades significantly. That has a direct effect on OBP (and basically every other offensive skill).
Bat speed does not just equal power. It's effect is wide ranging on a player's offensive profile. Once that goes, things can get ugly.