The Democrats had to be forced into "changing course" by a display of Biden on a bad day that was too egregious for any amount of whitewashing, and then they forced him out. Until then, they tried to paper everything over.
There's no "maybe" about Harris. She was a hopeless candidate during the 2020 primary. The administration soon got in the habit of hiding her away, and it wasn't because they wanted to protect her factory finish until 2024. In the early stages of this election cycle, one of the courses of action floated for improving Biden's chances was to replace Harris with another VP candidate.