Garrett never wanted to give up playcalling in the first place. Jerry made him.
Then when Jason stealthily stole it back from Callahan, Jerry had to slap him on the wrist and take it from him again. Jerry compromised by letting Jason pick his OC.
I have no idea who we'd get this time if that happened again. Garrett's list of fellow coaches is so short, there aren't a lot of qualified guys he knows. Maybe Norv is the Carolina staff gets blown up.
But the point is, Jerry obviously doesn't trust Garrett to make these important decisions himself.