There are polls rolling out with 45% of Democratic voters saying Biden should drop out.
Once you're at that point, it's over. I voted against Biden 4 years ago and was pretty annoyed that this was the best we could do, but I donated to him and voted for him. He won, he's tolerable, let's go. I would not have said "he's gotta go".
Now, I'm sure that same poll would find 10-15% saying Hillary or even Obama needed to be replaced, there's always "that guy". But 45% is way outside that margin. That's enough of an enthusiasm gap to doom the election.
If he's replaced, the other 55% aren't going to get all wacked out that we just removed the "best candidate". People will coalesce, independents will pile on, Trump will duck debates and whine, and whomever the replacement is wins in a walk.