What I've noticed about a lot of older players, is that while some can maintain really fast straight line speed, they lose their quick twitch, explosive lateral movement, cutting ability type of speed. A good example of this is T.O. during his last year here in Dallas. He was still super fast straight line, but his quicks were deserting him, and he wasn't nearly as effective.
Where this concerns Hershel, is that his strength never was being a side to side runner. So who knows? Maybe he could make a roster as a backup rb and kickoff returner? The guy has always been a physical freak and if anyone can do it, he can. That said, injuries would most likely put a stop to this comeback in short order.