You seem to think that someone other than Kamala Harris could have run for the Democrats once Joe Biden stepped down.
You should know that, legally, the only person who could have run after Biden stepped down was Kamala Harris. You should also know that as a condition of Biden stepping down so she could run, she was not allowed to deviate from his policies AT ALL during the election.
Oftentimes there are external constraints people come up against.
Should Biden have stepped down earlier? Sure. But that wasn't Kamala's fault. She was dealt a shitty hand and so were we. You do the best you can with what you have.