I'd love that. However, taking the reins for the remainder of this term, OFC, if Joe Biden were to step down as POTUS:
With regard to a replacement candidate, I don't see why she should have greater "legitimacy" than other top Democrats.
And when it comes to beating TFG, there seems to be only one person that can do it. And it is not Joe Biden. It is (drums) #MichelleObama who's ALSO said she would not be running this year: