That's a lazy argument. Outside of the fact that each man is his own and not a computer programmed by Belichick, it's a really small sample.
Romeo Crennel: very underwhelming as a HC.
Mangini: middling at best, but did enough to get a second shot.
McDaniels: probably too young, too green, and too headstrong when he went to Denver. But his failings were more in personnel than coaching. He alienated and ran out the QB right out the chute, and then the stud receiver; He then drafted Tebow in the 1st.
But the guy got the Broncos to the playoffs, and even won a game, with Tim Freakin Tebow. Should get an award for that. He was fired after only a season and a half anyway; you need at least 3-4 years to make a more definitive judgment on a guy. In his next go around, if given a GM, with the subsequent years of seasoning under his belt, he'll be good.
Bill OBrien: Has been solid with no QBs to speak of. Took a 2 win Texans team to 3 consecutive 9-7 records with 2 playoff births. Last year lost Watson which derailed the season. Now that he actually has a QB (as opposed to Hoyer and Osweiller), I think he'll be good. Knows what he's doing for the most part.
If you are going to include Charlie Weis doing poorly in college, then you might as well include Nick Saban, who might end up being the the GOAT CFB coach.
I dunno how Matty P will do. Time will tell. But they look like junk tonight.