I have no clue if he will be successful or not, and neither do you. Your premise is if he has never done the job before, you don't believe he can. Newsflash: Saban and Meyer didn't begin their careers as head coaches.
Again, I think GA should have hired a proven HC, but his predecessor whom most of us think should have kept his job wasn't a proven HC either when he was hired.
Fulmer, Richt, Jimbo, Jimmy Johnson, Dantonio, Cut, Bob Stoops, Chip Kelly...need I go on?
What's the percentage of success from assistants jumping to HC. That's a few. There have been hundreds of them. And most of the ones that worked out just kept the ball rolling at the school they were at.
And is Kirby's ceiling better than what Richt was able to do?
Most of the examples used were offensive coordinators that made the jump successfully and you can argue the reasons but the track records of d-coordinators making the jump have not been successful especially ones making the leap out from under Saban, and that is the key to the whole discussion.