Sadly, I think it's the fall from grace that almost invariably "completes" a fighter's career. Anyone who's ever retired on top has left unanswered questions.
Well yeah, but Foreman shut it down in his prime and came back a decade later as an old fat guy. A complete career should include a prime, no?
I can only think of 7 guys Rocky Marciano Lennox George Foreman Joe Calzaghe Floyd(if he wins out) Marvin Hagler Gene Tunney Marquez (If he retires today)