I just watched PBS Washington Week With The Atlantic. One of the members of the panel discussed the difficulty of finding a candidate to run instead of Biden, if he drops out, and especially given that the timing is very tight for a November election campaign. I think it was the same panel member who also mentioned the lack of success when Hillary Clinton ran instead of Barack Obama, although we know that the tricks played by Trump and the election interference made it more difficult for her to succeed.

Another panel member made the point that the Democrats can operate like a proper political party in discussions and decisions, even while weighing up the pros and cons of Biden continuing his campaign, unlike the Republicans who are "all over the shop" [that's my terminology], random and out of control.