Any democratic candidate is going to be dramatically unpopular due to the GQP personality cult propaganda machine, which also affects voters on the other aisle more than one might suspect. Biden has several things going out for him: (1) Not a woman, which means less of a reaction from the other side of the undecided aisle (2) He's stuck in the 20th century, like most of them (3) The fact that he's so senile and getting criticized for it is probably starting to get some sympathy from them because they are finding it relatable.
Hence why John Kerry is viable alternative. He's also a throwback, people who might not have voted for him both no longer like the candidate they voted for, Bush, who condemned the Trump era GQP and paid for it, and would still consider him a sign of times when politics weren't as divisive. He's also as old as Biden but not nearly as senile and a known figure. If the impossible happened and Kerry ran with, say, Mitt Romney, it would cement a candidacy that would show that the elections are no longer about party lines but about trying to stave off an encroaching personality cult fascism, that it is a fight for survival, and even more people would be inclined to vote for them.