While I agree with the statistical analysis, the majority of the RECENT hires have been specialists on one side of the ball.
Harbaugh - OC Playcaller
Kirby - DC
Saban - DC
Urban - OC and playcaller
Jimbo - OC
Chizik - DC
That's the thing with Mario I think he can do really well, but he's trying to win doing it a completely different way.
Never been a playcaller on either side of the ball, but he understand the box (i.e., run game and run defense) very well, and that's where supposedly his fingerprints are all over the team and game managing.
With that being said, he is an elite recruiter and a understand the box play really well. It would benefit him to always have up and comers be the OC side, so that he can utilize his knowledge of sound run game knowledge while allowing the OC to bring the passing game and proper flow of playcalling to the young gun. Its a balancing act. Meanwhile on the defensive side, let him do their thing with a focus on the run defense (which, in my opinion) is the right direction.
I think if Mario can hook up with a very sound and great OC (and successful in his own right, not a secondary playcaller like Gattis, who was at UM and Maryland), and let him do his thing, I think that's where he will fly, but it seems like he's having a tough time finding or acquiring them because they end up being HCs (i.e., Lashlee) and rightfully so.