I mean, his path to the office is in the line of succession laid out in the Constitution. Whether he'd be able to hold the Republican coalition together -- well, that's something else entirely.

Well of course, but what happens after that is the question. As you've laid out in this thread, I agree that there would be mass defections from the administration and a pretty rapid collapse of MAGA (and possibly the republican party is destroyed as well)

(Short form: Cults of personality are non-transferable; Vance lacks Trump's capacity to summon the threat of political violence that has given him much of his leverage; and while company men like Johnson will bend the knee, the True Believer freaks like MTG, Mace, and Tuberville will be wild cards)

Many Republicans are personally horrible people, but for the last decade or so the party as a whole has looked less like a coalition and more like a hostage situation. If Trump dies in office, or goes to jail, or is otherwise removed, that changes.

I think you'd see a wave of sudden retirements in the cabinet and the House GOP; for the real MAGA creeps, all the fun will have gone out of the game. Another effect: moderate senate Republicans like Murkowski and Susan Collins might risk the occasional vote of conscience.