There's both historical (as you have provided) and anecdotal evidence that rookie QBs and McVay don't mix well.
I think he expects so much from a QB, and his offenses are so QB centric, that to even consider developing a kid simply distracts from all the time and focus he wants to spend on developing an offense.
People can change. But right now he has his guy and there's no reason to think he has any interest in developing his replacement. Maybe a younger vet. But that's it.
