I’m not an insider, so I am in no position to say exactly what happened.
But my assumption differs from your assumption. I do agree that Biden had his arm twisted to drop out. So he dropped out. And I believe as a way to stab his party in the back he said, “I’m dropping out -—— and I’m endorsing Kamala Harris as the nominee.”
It’s very hard for me to believe that the party wanted to get rid of Biden, with a chance to start fresh, AND would freely choose Kamala Harris as their very best option.
But I wasn’t there.
As I recall, it took Obama a week or so to endorse her after Biden.
At that point the writing was on the wall. I don’t believe Obama thought she could win.
I assume Biden was pushed out by Harris supporters.
Sure, some other Democratic bigwigs might not have liked it. But they were blindsided by the Harris camp.