I have given this some thought for a while, but what if the Devil Fruit powers stopped working at the end of One Piece? Not that I think Oda would actually go this route, but Brook's soul would go the afterlife and Chopper would become an ordinary reindeer who can't talk.
Allow me to extend your idea:
Suppose, for whatever reason, Brooke and Chopper turned out to be the only survivors of Raftel. Now, Brooke is gone and Chopper won't be able to speak human language. No one will ever know what happened in Raftel!