He won because the Democrats care about whether their candidate stumbles across words and speaks incoherently, so Biden was pressured to step down
Have you time traveled from before the Biden vs Trump debate, good sir?
I do not wish to pick on one of your points because I am generally agreeing with you. But I must because you presumably neither seen nor heard about the Biden vs Trump debate that caused Biden to be forced out.
I am not even going to assess Biden's compos mentis. Maybe it was some medication or some benign reason, it doesn't matter. But what I can say is that his performance during the debate caused many of the die hard democrats to declare him incompetent and made it acceptable for media and pundits to turn on him (which they never did before).
To say that Biden was forced out because "he stumbled across words and spoke incoherently" is just blatantly false. Neither Democrats nor Republicans care if their candidate speaks coherently. Both Biden and Trump have spoken incoherently often enough and it has generally not affected their political career in any way.
He won because the Democrats weren't clueful enough to get Biden to fully step down and make Harris the next President immediately, which would have given the public months of seeing her actually lead the country.
100%. I want to reiterate that I agree with most of your points.