Nancy Pelosi was the main driver in getting Biden to step aside, but she did not want the nomination to default to Harris and advocated squeezing in an accelerated open primary. Harris polled better than Biden, but weaker than pretty much any other option. Nancy and Kamala never had a warm relationship.
If Joe Biden had flubbed up or passed out during the State of the Union speech in January, that primary might have happened. As it turned out, Joe looked fit and feisty in the SOTU, and the Dems really had no case to force him out.
Now it's looking like a different Dem candidate wouldn't have made a difference. Nor would a Nikki Haley or John Kasich kinda Republican done better by pulling the GOP back to a sane center. Donald Trump won by being Donald Trump, not in spite of it. He is the most influential American politician in our lifetime, perhaps even in our history. That's what's so horrifying.