I understand your point, but when the game is going on the coaches are talking about the next play before the previous play is even run. That's how they determine subs so quickly. This is the time the head coach can tell the coordinator, if we come up short here we need to run the ball or pass the etc. He doesn't need to be specific about the play if he doesnt want to, but he can determine the flow of the game. If the defense is tired, the HC needs to tell the OC, Hey let's slow it down, pound it a bit and milk some clock so our defense can regroup on the sidelines and vice/versa to the DC. The OC is just thinking about eh offense and the DC is just thinking about the defense. The head coach needs to take care of the offense when t we are on defense and take care of the defense when we are on offense.
I once scouted a game where the defense gave up a 12 play drive but came up with a great goalline stand on the 1 yard line. The offense went in and threw 3 straight incompletions taking about 20 seconds off the clock. Defense right back in. The DC went after the OC on the sidelines. The HC should have never allowed that to happen. Just an example, I'm sure Garrett does these things, its just that like everything else the bad call are always criticized.