I have no idea to what degree anything happened behind the scenes. Whether they begged him. Whether they threatened something. Whether they said, we'll give you one debate and if you can't handle it, promise us you'll step down. I have no idea. If I had to guess, people at many levels (family, Harris, Dem leadership) tried to convince Biden in private that it was in the best interest of all that he step down. And they should have found a way to be more forceful with it. But I would assume they did some calculus as well. If he wouldn't step down, your options are doing your best to put competent leaders around him to support, and trusting his word if he says he can do it. Or causing a cascade of events that look a lot like the internal party civil war between Gaetz and McCarthy that was an embarrassment.
This is once again a situation where it would likely really hurt the party with especially independents/moderates, were the opposition to put forward someone less polarizing.
I certainly wouldn't have chosen any of the way this happened. Of course not. I'm not thrilled about Harris. There are lots of things I am willing to say the Democratic party does wrong, or has positions I don't hold, etc. It is what normal people do. But, since everything that has happened was legally done so, I am left with the choices in front of me, which to me, is pretty clear. You disagree, which is your right. And if I thought you could admit your own parties faults (or admit your own misstatements), and weren't arguing against positions I don't hold, while I wouldn't like it, I understand voting policy over person and could respect the position at least.