Trump's issue is cognitive decline, it's sociopathy.
I suspect that it originally, but now there is some significant additional dysfunction on top of the sociopathy/megalomania.
That's one of the things that is terrifying about a hypothetical 2nd Trump presidency (not that he is legally able to be President, but Republicans care nothing for rule of law). I believe that if Biden truly were incapacitated and unable to do the job responsibly, his Cabinet would step up and remove him if he wouldn't resign (and I expect he would), and that the Democratic party would abandon him should he attempt to stay in office. I fear that, based upon demonstrated behavior in the past, Donald Trump could go full
Downfall, non-stop and nonsensical raving insanity, and his cabinet and party would just obey and make excuses. (Some of our Trumpist posters may claim this is already happening with Biden. To which my response is: it is always projection.)