Not necessarily. Trump could still come up short since Cruz and Kasich would still be on the ballot. And by the standard you have already set for Kasich and Cruz, Trump should immediately withdraw in that event.
Fully agree he should withdraw if he’s mathematically eliminated. After all, both Kasich and Cruz have said that’s the right thing (heck, in Cruz’s case, that’s the moral thing) to do.
The fact they don’t practice what they preach is incredibly telling. They are both hoping to be anointed by the GOPe rather than being the choice of the people.