What if they're throwing it? They can read polls as well as the rest of us and they know they're not going to win. They don't have any good candidates ready and even if they did that candidate would have the Biden dumpster fire as an albatross hanging around their neck for their entire campaign.

It could be a rational choice to let the senile fuckup take the blame this time, spend four years fundraising to save Our Democracy™ and then run a Buttigieg or whoever it is they really want. At that time people will have forgotten how awful Biden is because they will have been busy with how awful Trump is.

Letting Biden stay on long enough to lose even comes with the added benefit that they get to pin his unpopularity on his dementia rather than facing the reality that their policy is dogshit.