YES. However, it is very hard to know when to step down. When you are on top there is always someone new out there challenging a top fighter on his way out. im sure we can all think of plenty of fighters who had this trouble.
I personally tend to reflect on the peaks of ones career rather to penalize them for carrying on beyond their best days.
No. If it does, then very minimally. It's the norm for great fighters to go on too long. Ray Robinson, Muhammad Ali, Evander Holyfield, Roberto Duran, Ray Leonard, Alexis Arguello, Julio Cesar Chavez, Ezzard Charles, etc.... No one says Robinson fighting until 1965 prevents him from being the greatest ever. If they have him below #1, it's for other reasons. Same with Ali. Does anyone say that Michael Jordan's stint with the Wizards prevents him from being the greatest b-ball player ever? It basically just messes up your last memory of the athlete, but doesn't take away what they achieved and how great they once were. I expect boxers too fight on too long until they are far from what they once were.