Biden could straight up choose and endorse someone. Even if it wasn’t Harris, which would look kind of shitty, but the DNC isn’t completely stupid. They’d fully back whoever they decide that Bidens endorsement (see what I did there?) would be. It’s too late to let the dnc campaign and come up with a popular vote. It isn’t too late for a presidential endorsement to work.
I think if the public has a choice between an old crazy liar or an old senile guy, or literally any competent 50 year old backed by one of the two parties.