I am not so sure.
The reasons given that people are okay with Trump is that he stirs in them a new patriotism, a feeling of kicking ass, a relentless negotiator to make America great again.
I do not see any of that in the presence of Jeb, so I do not see Trump supporters going to Jeb should Trump drop off.
Would they disperse though? Leaving jeb with enough moderate votes? That’s what frightens me.