Ummmmm... Just thinking about this. He still has 13 days in office, and god only knows how much more damage he can do.
But, just hypothetically speaking, let's say that they invoke the 25th amendment and it goes down in history and on the record that Trump was mentally incompetent to hold office...
Now I'm just thinking out loud here: He leaves office and he starts getting indicted for a tonne of crimes, both state and federal.
If his defense team could prove that he has been mentally incompetent for ages, then they could have a pretty good case for an insanity plea!
Basically stating that he never had the mental competency to understand the consequences of his actions.
Amendment 25 would set a precedent, and his defense team would just have to prove that there has always been a pattern of this type of behavior.
Edit: Scratch that! They're circulating articles of impeachment in the house as we speak!