Bernie will beat Trump in the general, but would lose to virtually every other Republican. Biden or Warren will steal it from Bernie first.
The only way I see Biden and/or Warren coming in is if Hillary wins the nomination and Obama takes her out with an indictment. (Later to be pardoned, of course)
If Bernie wins, I think, there is no way to push him aside in favor of someone not already in the race.
The President is in the catbird seat. Which is in Obama’s best post-presidency interests? Not a Clinton or Sanders presidency, I’m thinking.