nfl

That's definitely some of it. But backs can get better (or at least more prominent) through their careers. Thinking Marlon mack, pretty much every 9er RB, even Henry. Reading through the list of older RBs is a shocking list of former greats though. So many who used to be stars and now can't do more than bit parts.

But also replaceability. It's so easy to find running backs who can be effective cheaply either in the draft or free agency. The latter is self perpetuating and related to the above, teams shipping out good older guys because they don't want to commit to long deals with older guys. I suspect the former has to do with how many of the best athletes play RB at lower levels so lots of pro guys know how to do it.

Then there's positional value: the importance of running has diminished and you only need 1RB on the field compared to say 3WR typically or 5 oline.