Most Biden supporters will ‘vote party’ with or without Biden.
Yes, his supporters will but the number of supporters are dropping.
Neither party can win with just their core supporters..
Likely, but if you go read those posts over at that Young Turks vid I posted a link to, the Dems are split 3 ways, roughly equally. ~1/3 will vote blue no matter who “blue” is running. ~1/3 think Biden is a war criminal and that will apply also to any of the likely replacements. The last ~1/3 are not all totally committed Dems, and regardless now see Biden as unacceptable, and just want someone younger, but again, none of the likely replacements will satisfy that middle group. The “he’s a war criminal” types have nowhere to go in this election. They may vote for local Dems, such as AOC, who are anti-Israel, but they’ll likely sit this one out or vote for some minor candidate for President.