That's the issue. Barring some other explanation, the fact that he is incoherent like that even some of the time is immediately disqualifying. You can't roll the dice on him being compos mentis when an international crisis occurs.
Presidents (including Biden!) have formally invoked the 25th Amendment just because they were being briefly sedated for a minor medical procedure for an hour or so. Somebody's gotta have their finger on the button at all times, we did a whole constitutional amendment about it.