I won't pretend to know how it would have gone either way. Had it actually not happened in the timeline that it did happen, it could have easily been different. The momentum, the media narratives, the quickness with which the Democrats united behind Harris, all of these things caught most everyone by surprise. I could see any of those things being wildly different had Biden chose to step down much earlier. We don't like to admit it, but irrational, unknowable elements like "vibe" and "timing" have always played a huge role in elections.