He's made his place at the sport's highest level, along with the all-time greats. He has his money. And his body is telling him that it is time to get out.
That said, the real question is how will he feel in 12-24 months after retiring? Will Floyd experience what the boxing calls "old fighters' disease," where he watches some young lion who is entering his prime, and think, "Shit! I'd kick his ass!" ? Will he be able to adjust to not being the exact center of attention -- the position he has enjoyed for several years now?
I'll also add this: Berto will give Floyd a much better fight than did Pacquiao or Canelo. He will perform more like Ricky Hatton did, in that he will give it 100%. He'll roll the dice, rather than being satisfied with going 12 rounds without getting knocked out.
While I don't think he can win, I also keep in mind that when others have attempted to reach Marciano's record of 49-0 (which, as I have documented, is a myth, as Rocky lost 5 early pro fights that his manager "removed" from his record), funny things can happen. Larry Holmes, for example, picked an easy tune-up fight with a light heavyweight champion -- formerly a common practice for a pay-day for both -- but lost to Michael Spinks. Highly disputed decision, but a loss. A motivated underdog with nothing to lose can be dangerous, for an unfocused champion expecting an easy win.