However, the Democrats don't want to try triggering the 25th Amendment because it would look even worse than it already does. The fiction will be that although he's still capable now, they don't think he will be capable a year from now, or two years, or whatever it is.

Pelosi clearly doesn't want Harris as the candidate and is getting Schumer behind her, but the Democrats are really royally screwed no matter what and the candidates who could challenge her don't seem to want to. And why would they? The Democrats are facing the strongest headwinds in years and Trump can only serve 1 term, so it's better to sit it out. Otherwise, they have to try to deal with, and on a truncated schedule:

1. Israel's ongoing Gaza campaign will mean all the whiny Leftists go back to protesting once they get back to classes. Support them, you lose the liberal Jews. Support Israel to keep the Jewish vote, all those Leftist kids will vote RFK Jr. Wait four years, and maybe this mess will at least not be in the forefront of people's minds.
2. Ukraine very well could collapse before the elections. Sure, you can blame Republicans, but it's becoming obvious that the media lied on this, as in so much else. Ukraine could never win short of total NATO intervention. By postponing peace negotiations and actually refusing to engage with Russia in any meaningful way from November 2021 on, that debacle will also be on the Democrats.
3. Speaking of debacles, you have to distance yourself from the total failure of the administration in getting out of Afghanistan, which is the most humiliating failure of the US military in my memory, the 8-10 (or more?) illegal aliens that have been allowed to flood into the US, the ongoing rampant inflation, the lawlessness in American cities and pushing unpopular things like allowing men to compete in women's sports by pretending to be women.
4. Trump is more popular than ever following the assassination attempt. That one photo of him, bloodied, fist upraised and framed by the American flag, is better than any campaign ad by several magnitudes of importance.
5. There's no enthusiasm in the Democratic Party and trying to generate some will just be forced no matter who the candidate is.

So if you're Gavin Newsom, or Gretchen Whitmer, or any of these would be Presidents, why would you want to go up against near certain failure NOW, rather than just wait 4 years and build up your support network, your message, and let Americans get pissed off from media coverage of Trump again?

In this case, it's easier to let the scapegoat be a Camel Toe.