The thing is that I think Kubiak is a solid OC but not someone who is irreplaceable. I think he's still very much so the same guy he was when he was the Vikings OC. He just seemed better than he is because we are comparing him to Pete who was not running a modern offense.
Of course I also don't want Gruden.
I think if you want to develop Rattler you go with a young offensive head coach who has developed young QBs in the past and runs a similar system. i.e. Ben Johnson or Slowik. And I'm sure there are other young guys in the McVay/Shanahan/Kubiak tree that would either work as HCs or OCs under a defensive head coach.
The truth is that we don't really know if Kubiak can develop a young QB. But we do know that Johnson did a good job rebuilding Goff and Slowik did a good job with Stroud.