You're underestimating just how much contempt there is for Biden. He's losing to a Trump even more now after the debate, absolutely hemorraging support. He simply cannot win. He's alienated the anti-genocide part of the party, calculating that he could afford to give them the finger if he tried to suck up to independents and non-existent disillusioned maga chuds. And now independents are abandoning him.

There's no evidence that the party would be split on a different candidate, the only argument I've heard for Biden up to this point is "Vote blue no matter who. He's not Trump.", which can be applied to literally anyone. Even Harris has a better chance against Trump at this point, the numbers show that.

The anti-genocide people do not want Trump, but Biden literally made it impossible for them to vote for him, they will vote for basically anyone else if given the chance. Harris even has the benefit of having distanced herself from Biden's insane support of Israel, and she's a woman and not a catholic so she's going to be stronger with women on abortion, she's also the reason Biden has any of the black vote.

Biden is a sinking ship, the life raft is not great but it will work better than just staying onboard and going down with him.