95% of the Democratic platform is that Trump is dangerous and a threat to democracy. Trump can now respond “Democrats literally tried to kill me but yet I’m the threat” and win any argument on that topic. It’s an absolute lie but no one, especially low-info voters, care at this point. It may not switch any voters but it almost guarentees that a lot more Republicans will vote and clueless moderates will think less of Biden. Combine this with Biden already being a absolute embarrassment and it’s not hard to see this as a deathblow.
Plus, the optics of the assassination were fucking fantastic. That fist raised photo will literally be in history books for 50+ years. Trump may not be competent, but his team never lets a good PR opportunity go to waste.
I think she has the best chance of winning but ironically she has the same major liability that Joe Biden had, that is being associated with Joe Biden.
Almost any attack Trump could make against Biden, barring being senile, works for Harris because she was literally in the administration. The portion of the populace that votes to punish the current president for economic downturns will hate her almost as much as Biden.