If Biden had been 25th Amendment-ed, and they could show cause (dementia), I would have been OK with it. That's why we have VP. I voted for Biden in 2020, but it would have been for the good of the country if Harris had taken over a year, or maybe even two, earlier.

I think that's slightly different. There's no moral consequence to developing dementia - you didn't do anything wrong! - nor is there any damage to the President's political party if he is discovered to have developed dementia and has to be removed. Or if he were to have a stroke and be unable to continue in office. That's not going to hurt the party (other than perhaps resulting in the loss of a capable member of the team). There's also a certain unavoidability to the issue - if the President has dementia, there isn't a single moment that if he gets past that moment, he'll be okay and be able to continue. If you vote against the 25th Amendment on a Monday, the President will still have dementia on Tuesday, and the issue never goes away. And you're not handing the other party a scalp.

It's different if you accuse the President - the most important member of your political party - of actually engaged in criminal activity. That's bad. It's generally been bad for the party if it turns out the President actually committed the criminal activity (see Nixon). It wouldn't be, "shame about the former President's health, let's move on to the next person"; the President's moral and legal failings would be a cudgel against his party for at least the next election cycle or two. And unlike dementia (or a stroke), the President could get past it as long as everyone plays along. If you don't vote to convict, the impeachment dies, and the President just moves along.

To say nothing of the fact that there's no "surprise" here. Trump's venality is enormous - but completely lacks subtlety or artifice. He is brazen and shameless, and does virtually everything in the open. He's a "broad daylight" kind of guy, not even bothering to conceal what he does. Which means almost every Senator is basically already committed to the idea that this stuff isn't criminal. For example, his mix of family business with foreign policy? It's been widely known for months, and no GOP Senate incumbent that's making it to Jan. 3rd has condemned him for it - so now they're going to vote to find it to be a "high crime or misdemeanor"?

There's no way it helps a GOP Senator to vote for conviction. Whether it's the right thing to do, whether it's for the good of the country or the good of the party at large, too much of the GOP base does not want to see the Democrats get a scalp - and will resent any Senator who gives it to them.