I'm offering my opinion on the answer to the question of why (basically post debate) Democrats think Kamala (or someone else) is a better candidate than Biden. The number one goal (for most) is to win elections.
So, you are forgetting that they were lying and gaslighting about Biden being so great and never in better shape to serve another four years. Dumb.
DNC didn't even allow for a primary and debates.
Post debate Biden odds looked to drastically change (and with it being health related it wasn't just say a dip in the polls that Biden could come back from, his health isn't going to do a 180 - it's permanent) and the call went out (for him to leave).
Lie and stupidly lame. He was looking that way during the 2020 DNC debates. He's been shuffling, losing his way off of stages, taking the kiddie stairs on AF one and bloviating gibberish since the election.
Duh.