I think that Harris was a very poor choice of a candidate. I have no way to know this, but I like to imagine that a better Dem candidate would have led to a different president.
I know that I've probably been wrong ten times for every time that I've been right about something like this, but that didn't make it any easier to watch the last six or seven months. You're correct that there was nothing else anyone could have done to avoid the train wreck by the time Biden withdrew because the only way to avert it would have been to change speeds way back before we got to the sharp curve. The worst part is that I don't even think that Harris would have necessarily been a bad candidate if she had been given a proper chance from the start; there are articles as far back as early 2022 mentioning that her public appeal was hurt by the Biden administration tying her to issues like immigration and election reform. At minimum, ignoring the harm to her future electability showed extremely poor foresight, and it's not hard to imagine that it went beyond negligence to the point of outright sabotage of someone viewed as a potential threat.
In the end, I think his policies turned out to be great. But he didn’t have the skills to capitalize on the gains, to seize them and point to them as “wins”. Instead we often saw a somewhat slow, very old but good natured grandpa who was clearly not fully present all the time.
The policy on Gaza though, that made 0 sense. He had the power to bully Netanyahu into stepping back but consistently took no action. Solving that crisis would have been HUGE for reelection, keeping them festering only contributed to the feeling of chaos.
That would be landslide GOP, California goes red bad.