The main reason not to use it is that the real chief complaint against Donald Trump is that he threatens U.S. democracy not (chiefly) by breaking laws, but by undermining the norms which are just as important to democratic governance as the laws and constitutional provisions. And therefore efforts to remove him should be especially careful to abide by those norms [bold mine-DL]. The 25th amendment is for use in Wilson-like cases where the president is really, truly incapacitated. While mental illness could qualify, the many armchair diagnoses we’ve seen of Trump simply do not clear the constitutional bar.
Invoking the 25th Amendment in this case wouldn’t just do violence to norms, but would make a mockery of the plain meaning of the language of the Constitution. Trump has demonstrated remarkable incompetence, but he is not so physically or mentally disabled that he can’t discharge the duties of his office. He may discharge those duties badly, but that is an entirely different question and one that the amendment was never intended to address. Going that route would also require more support in the House than an impeachment vote would, so it would be even less likely to “work” in removing Trump. It would be an illegitimate use of this part of the Constitution, and most members of Congress wouldn’t support it anyway.
If there is a need to remove a sitting president, impeachment by the House and conviction by the Senate are the most appropriate means available. If Congress can’t or won’t use those means, the only other acceptable alternative is to persuade the president to resign of his own volition. Under present circumstances, the latter method might be the most likely to succeed. The temptation to abuse or skirt constitutional rules to remove a bad president is always present in any democratic country, but it is something to be resisted. If we start giving in to that temptation, we really will end up with a banana republic. The damage to our constitutional system will be far greater as a result than anything that one bad president can do in a single term.