I think this is blown out of proportion. A fighter (or athlete) going on for too long doesn't take away from the achievements he had while in, or near, his prime. It may mess up your last memory of the fighter, but that's about it. Most great fighters suck at the end of their careers.
I couldn't possibly agree more. That is almost as absurd as people saying Jordan tarnished his legacy by playing for the Wizards. The last time I checked he still has 6 rings at the end of the day!