Not sure how to answer this. Not sure how to be scientific about all this.
I believe I was careful with my language regarding a ceiling. A given roster has a limit to what it can accomplish. That limit may be higher than was realized, but generally, teams flying higher than people thought they could fly do reach their limit. I think we see this every year.
Your challenge to find counter-examples seems hard for me to get hold of. The QB position is such a dominant one in the NFL that it's hard to think of team success without good QBing. And yet, I can think of examples.
Those of us who remember the Rams of the 80s remember very well coached teams that achieved at very high levels without good QBing. Was John Robinson a great coach? I think he was borderline--a superb leader with a clear vision who had a lot of success without having the final piece of the puzzle. Those Ram teams played pretty damn close to their ceilings but couldn't overcome the Pat Haden/Dieter Brock syndrome.
But I'll cite another example: Vince Lombardi. No one questions Vince's greatness as they think of the Pack. But of course, he had Bart Starr. And yet ... I actually remember his move to the Wash. team. That bunch had been moribund for years at a time when a team had to build over several seasons through the draft. I remember thinking, "Lombardi won't be able to do anything in D.C." And damn if I wasn't wrong about that. Lombardi led D.C. to it's 1st winning record in 14 years. He then died of cancer, leaving the team George ALlen would succeed with.
Now, who was the QB for Lombardi? Uh, Sonny Jurgenson, a Hall of Fame QB. Does this disprove my point? I don't think so. First, the league was smaller then and almost everyone had a good QB, even the bad teams. And that's the point--Sonny had been there for 5 years accomplishing nothing. The QB couldn't lift the team. But a great HC stepped in and virtually instantly it became a winner.
One more--Brian Billick with BALT. The man won a Super Bowl with 2 QBs of note: Tony Banks and Trent Dilfer. Yeah--Tony Banks, who actually had a good year. Of course, Billick did have great players on that defense, but not QBs. I don't know that Billick was HC long enough to be considered great, but he surely raised that team above its apparent ceiling.
Again, as I say in my response to LA, I don't view coaching greatness in terms of W/L records. That makes it hard to achieve coaching greatness in the NFL without multiple rings over a long time. I do think there are great coaches at lower levels who achieve a lot with a little.
I guess I'll step back from the word "great" a slippery term fraught with ambiguity. I'll simply fall back on what we see in the league all the time: coaches who fail to raise a team to its ceiling or lose the edge they once had when opponents figure them out or they burn out. Coaches in the NFL either galvanize their rosters for success or let them slip into a malaise of failure. We have watched a once decent HC gain marginal success in his 1st year with us and then slide, year after year, into ineffectiveness. We've watched a team that had some decent talent go into games and even seasons unprepared. We've watched them play without discipline, waste chances to win, and fail to believe enough to make plays happen. We've seen numbers of individual players display the same signs of uncompetitiveness: mental mistakes, botched execution, and confusion.
I personally don't see how to put all of that "on the players." There have been too many players screwing up too often. I see palpable signs of coaching malfeasance in the performances on the field.
But I understand that other fans tend not to see the hand of coaching in all of this. That seems very strange to me, but people see things differently.
The test will come this fall. Can McVay and Phillips lead this roster, including many from last year, to play better, to approach its ceiling? If they do, we'll know most of it was coaching. If they don't ... well then we'll keep arguing as we try to figure out whether McVay was a mirage or whether the roster just isn't any good.