I think you're missing the most important focus of his -- himself. His biographers have stated on multiple occasions that the man is by far and away a self-centered narcissist. If he believes in himself that much, I don't see him dropping out barring him being rendered physically unable to continue.
Narcissists quit when the situation is too painful for their ego to bear. If it gets to the point where he's being raked over the coals and there's a consensus nationwide that he's a horrible candidate and will absolutely lose by a wide margin, he might very well quit. I guarantee you the daily attacks on him by Larry, Trevor, and other popular young commentators really hurt, because he's a thin-skinned egotist.
He could insulate himself in his cult, play to adoring, drooling crowds, continue making the outrageous remarks they love. But the possibility of being completely humiliated in November would weigh heavily on him.
