The grad transfer situation is now a fact of life in college basketball. North Dakota for example just lost one of their better players to Gonzaga.
Having a good relationship with a coach or coaches helps but doesn't mitigate the possibility of talented kids leaving. Playing time could be part of it, a chance to measure yourself against "better" competition, perhaps a chance to play in the NCAA Tournament. The reasons are to numerous to lay out.
It is what it is. Personally I wouldn't worry about ISU kids leaving in the future, if it happens, it happens. You can't stop it and basically you can't control it. You do the best you can to establish a good relationship with a player, you hope they show some loyalty and let the chips fall where they may.
Besides as has been pointed out in the past, and it does have a ring of truth to it...basketball coaches leave at the drop of a hat contract be dammed. Athletes look at that situation and think, "why can't I?" just like any other college student, especially since they have already graduated which is a requirement for being a grad-transfer.
Perhaps the best way to insulate yourself against this happening is to start winning games, that usually cures all issues from attendance to booster money to dissatisfied athletes. (I know easier said then done sometimes...)
PBP