For one, Biden would have to willingly give up the nomination in order to release delegates already pledged to him.
Factually not true…
The DNC can rewrite the rules before the convention and nominate anyone they want.
As their lawyers have told judges, they’re a private organization that can name anyone as the candidate. That was their defense for accusations that they already rig primaries.
A second problem is the public doesn’t know any other Democrat nearly as well as they know Biden, and it would be difficult to introduce someone to the public at this late date without them being defined by Donald Trump, the Republicans and Fox News in the worst possible ways.
Literally anyone that becomes the Dem nominee will have nationwide name recognition in 24 hours.
The most common reason for voting Biden is “to stop trump” so any replacement shares the most important quality Dem voters want that Biden actually meets.
Like, sure, this would have been so much easier if we had an actual primary and the people closest to Biden hadn’t spent four years hiding him and lying about his mental capabilities. But they didn’t. They kept repeating it was too late for anyone except Biden.