It has to be believable. Even if it were true in your hypothetical, Xi would not believe that Trump or any other leader would sacrifice their life and the exist

It has to be believable. Even if it were true in your hypothetical, Xi would not believe that Trump or any other leader would sacrifice their life and the existence of the entire country over an issue that isn’t of existential importance, either to the leader or the country. That’s, again, the whole problem. You don’t have to view Putin as “mad” to see him taking a nuclear last stand over Ukraine. He is already invested and taking risk at an extreme level to the point that no one can doubt the importance to him and his willingness to make heavy sacrifices to win. There is no believable narrative where a US leader would be willing to gamble at an existential level over Ukraine.