AZ will likely be a dumpster fire unfortunately for him....they are, besides the Texans, the worst run and most irrational organization in the NFL. There is a reason why the Texans considered him last year and AZ hired him this year: they have no clue what they are doing.
I know every coach wants to become a head coach and should never refuse an offer, but one needs to be smart about their career IMO and failing as the head coach in AZ will be a mark on his record. As a young white man, he'll get other opportunities, but there is only one chance to make a first impression. Ask Kliff Kingsbury about that.
I'm mixed on Gannon as DC here. I think he had plenty of talent at each level: line, middle and secondary to work with and his scheme of negating the big play with 2 deep safeties, corners playing off and mostly zone looks worked pretty well against flawed teams that cannot run and pass effectively. It also combined well with an offense that quickly seized leads and kept scoring until the opposing defense tapped out and allowed the defense to attack the QB without much worry of a run game.
The problems arise against the elite balanced teams like KC who can run the ball against our mediocre run defense & pass effectively; especially in the short to medium range to continue moving the chains and score consistently. They also have a defense that can hang in for the game and will not tap out easily. The biggest issue is we could never build the lead large enough to turn that tide and convince KC they had to abandon the run and start taking bigger risks to score. KC stayed patient and methodically broke our defense down once they had confirmed the tendencies Gannon was showing them. Once KC scored the TD with the first drive of the 2nd half, the Eagles needed to respond by scoring a TD to keep the 10 pt spread. Once they settled for the FG, I suspected we were doomed.