Back then, the 49'ers were running the standard West Coast Offense, so you had to have an agile running back who could run with power, evasion, and speed, while being a legitimate pass catching threat.
They had some fantastic years with Roger Craig as the featured running back, along with Tom Rathman being the bruiser from Hell.
Once Craig started slowing down, they tried other backs, such as Dexter Carter (too small, couldn't run with power), Keith Henderson (medicore at everything), etc., but nobody could fill the shoes of a Roger Craig in his prime until Ricky Watters came along. He would have made an impact in 1991, if he hadn't broken that bone in his foot during the preseason.
Once he left, the 49'ers tried backs such as Johnny Johnson (injury hobbled) and Derek Loville (mediocre at everything), but there was an obvious dropoff.
Unlike other backs, he actually got better with age.
He would have certainly been in the Hall of Fame if he didn't say those four forbidden words, during his time with the Eagles. That was all on him, the fact that he mans up to having been inappropriate like that earned a lot of respect in my eyes.