What every team needs as a HC, a man that hires great coaches. That's his # 1 job just as the GM's #1 is to hire the HC and the owner's #1 is to hire the GM.
It is a chain, a chain of command, and when interrupted in the flow, it kinks.
Sean McVay, the youngest HC in the league is smart. Look at his coaching staff. A smart HC hires coaches that know what he doesn't know. A new HC hires the most experienced and successful coaches he can find.
Garrett is a coach without a pedigree. And who does he bring in? Except for Kiffin, where are the skins on the wall? His two coordinators are two of the worst HC's in NFL history, one an 0-16 coach.
The most questionable hiring since Garrett was hired is Linehan. Just what are his credentials? 5 years at DET leading the league in pass calls and how much did his young stud QB progress? A hell of a lot more after he left.
It's not the HC, it's the staff. Belichick doesn't have the best talent on his OL so what does he do? Gets the best OL coach.
Except for Parcells, and that was for an ulterior motive, the GM of the Cowboys has done a very poor job of filling the most important element of any football team.