"Any iteration of a Democratic president (including a mentally incapable one) is preferable to Trump" is an extraordinary claim, that's why I can't agree with it, and I don't imagine many other people will.
I am much more ideologically aligned with the Democrats than with the Republicans, and I give a lot of priority to ideology, so it's basically impossible for me to prefer a Republican administration over a Democratic one, in broad terms. But politics isn't just about ideology. There are matters that are apolitical, matters of 'competence', 'capacity', etc. Even for someone like me who prioritizes ideology, I cannot go as far as saying that a mentally incompetent person who is closer ideologically to me will always be better than Trump. It's probably true in most cases, but I think you are seriously lacking in imagination if you think the "worst case scenario" of having a mentally incapable president is literally nothing.
I suspect where we disagree is really on the matter of competence. I don't think the Trump administration was dramatically incompetent and I don't think the Biden administration has been dramatically competent. Beyond disagreeing ideologically on things like Gaza, you can't possibly look at that and think "they're doing a great job by their standards I just disagree with them."