The "It's got to be an offensive coach" argument is weak to me.
There's an obvious problem. If every team is only looking to hire offensive HC's there can never be enough qualified offensive minds with the skills required to fill those roles. So you end up with Hackett's, Gase's and Arthur Smiths's as head coaches. You end up hiring 4's from the offensive side of the coaching pool while a bunch of 8's, 9's and 10's from the other areas of football languish.
As far as I can tell only 13 teams had play calling HC's last year leaving 19 that didn't.
Look at this list of current OC's. How many of them scream head coach?
en.wikipedia.org
Steichen - 37 - 3 year OC
Callahan - 38 - 4 year OC but doesn't call plays
Bieniemy - 53 - 15 failed HC interviews and doesn't call plays
Kafka - 35 - 1 year OC
Johnson - 36 - 1 year OC
That's the best 5 candidates from the 32 current OC's. There's no McVay's, Shanny's or Reid's there. Of the 5 I'd bet at least 2 are Kliff's and 1 is a Gase. EB is by far the most qualified but 15 failed HC interviews happens for a reason.
It's not exactly a great bunch.