In my opinion, Frank leaving has as much to do about wanting more PT as it does with JB/his ways/system, etc. We all know, if Jesse wasn't a foul machine, and could keep his arse in the game, JB would play Jesse 35 minutes plus per game. JB has pretty much always played his starters excessive/heavy minutes, and there's no way JB is making an adjustment to his MO, certainly not at his ripe age of 78. Unless, he's absolutely forced to due to extenuating circumstances beyond his control, i.e., foul trouble, injury, etc.
Frank is well aware of this. Therefore, Frank in all likelihood, came to the realistic conclusion that if for some reason Jesse figures out a way to stay out of foul trouble, his minutes could very well go down from this season's. So, with players having far more flexibility nowadays in leaving/transferring, etc. without penalty, he's betting on himself for greener pastures.