It's primarily an ego thing.
If Trump wins again now in a rematch and gets a 2nd non-consecutive term while their guy dies forever remembered as a 1 term loser (Let's be real: Biden's body, mind or both ain't making it to 2028), that's arguably more humiliating than if Trump had just won 2 times in a row, and they know it.
I don’t think it’s an “oh no I’m owned” thing, more that they need to feel like the country is being sensible. Like, yeah, ain’t got shit to do with actual policy passed. That’s the irony, the people who fret the loudest about the dire fallout that’s going to happen if Trump gets a second term are the ones least likely to suffer material consequences. For them, Team Blue winning is a totem of normalcy, a manifestation of their underlying assumption that the country doesn’t want anything to significantly change.
Yeah, I think it's this. They want a well-spoken guy in a suit calmly explaining things, they want the news to say this guy's policies are good, they want the occasional fiery speech but only in moderation and when the news says its for an important crisis. The policies don't matter in the slightest.