Harris will be the next Dem nominee whether she replaces an institutionalized Biden in '24 or has to wait until '28 if he hangs in long enough to be beaten in the next election. The Dems are now victims of their own PC-ness, and any attempt to pass the crown to someone else, unless it is another female minority, will be looked on as racism/sexism within the party.
I don't think they were actually expecting Biden to win when they installed her as his running mate for show purposes, but now they're stuck with her.