You say that he should have "completed his apprenticeship with us" then moved on, as if it's a given that a club like Leeds would come calling again a few years later?
As I said previously, that's not how professional football works. It's not a three or four step program.
What if he'd stayed with us and not made the grade? Or run into a similar issue as the one he faced when here on loan? A manager who didn't fancy him and a team that was shit? We've happily cut loose players like Corey O’Donnell, Daniel Hunter, Kian Speirs and Logan Dunachie without a second thought. What's to say McKinstry wouldn't have ended up suffering a similar fate?
Nah, when a club like Leeds come calling, you make the move. Any "apprenticeship" he could complete playing in a Motherwell team in the SPFL is no better than working with coaches like Marcelo Bielsa and training alongside the kind of players Leeds had, in the facilities that Leeds train at.
Football is a short career. You make the money when you can, because once it's all over and done with, it's the money that will be paying your mortgage or setting you up for the next stage of your life.