You can't cut Carr next year, so who can fix him?

McDaniels isn't a great OC, he coached the best QB in NFL history. A good OC would build a system that wasn't too complex for the most important position on the field. They would build one that suited that player. McDaniels thinks he's great and isn't. Gruden designed a system that fit Carr and Carr had his best years in the NFL as a result.

Your last point is actually really good. He isn't a good fit for a system that they won't alter to fit the players they have. That was literally the reason they had Gruden come in during camp, to help them fix that. Unfortunately, that died when he left the building and Pete went back to what he knows. Which IS the problem we've had with all of the QBs over the last two years. A refusal to adjust to the players we have, and coaching as though we have the players we want.
That scheme is actually butter once you get it because of how flexible it allows the QB to be. But he is a good OC and I believe most would agree with that consensus. He had Mac Jones looking like one of the next best things his rookie season. And that offense was 6th in scoring and 7th in red zone conversion rate. That's how he got he Las Vegas job.

And while Carr rewrote his story with Gruden, it shows that most of the league really care little about substance and more of the appearance of production. Carr was racking up yards, completion percentage and near the bottom of the league in TD%. That was the first thing that caught my eye more than anything and diving into that is how I tied that to how bad he was in the red zone during the Gruden years. I think he adapted better to Gruden's system because it was closer to what he was doing under Del Rio/Musgrave but that still took time.

That's why I always talk about perception.