if she was the one who ultimately succeeded in getting Biden to drop out when he did, then isn’t it reasonable to expect that he would have dropped out earlier if she had pushed him out earlier?

No. His debate performance is what pushed it over the edge. That’s when a concerted effort began to get him out.

This is naive. A Democrat isn’t going to challenge a sitting president for the nomination. Pelosi is absolutely right about this. Biden never should have run for reelection.