I've thought a lot about this. Can we say that an HC can be responsible for a QBs regression?
I suppose one could make a case for trying to pound a square peg into a round hole; that is, an HC trying to make a QB something other than what he is. Trying to make the QB fit a system instead of designing a system around a QB. Although, we all know it's not quite that black and white.

But the 2 glaring examples currently are Wentz and Goff. And they've had the same HC. And in both cases the HCs were offensive minded. In fact in the case of Wentz, his HC was an NFL QB. And they have bother regressed noticeably in the past 2 years. They were good enough to get big buck extensions. But the.....pfffffft. Down the drain. Can that really be blamed on the HC?

Wentz's case and Goff's are a bit different imo. Wentz's has much (not entirely) to do with injury. Wentz's game is such that he is somewhere between a mobile and a running QB. And it is because of that aspect that he got injured. And now he's a bit gun shy to run, and also has decreased athleticism due to those injuries. Result: his game has taken a hit.

Goff's regression is the more befuddling one to me. He simply went from playing well to not playing well. He became a turnover machine.... of every kind. He couldn't seem to get through his progressions. Even many of his completions were somewhat errant. And some of his incompletions were head scratching as they were easy passes for even a high school kid.

We've cussed and discussed the reasons why for Goff.
But in the end, it seems to me that in most cases the regression is on the QB and not the HC. Perhaps it also involves Defenses figuring these guys out and knowing exactly how to play them (the way the Rams have figured out Russ). Exposing a serious weakness (such has Goff not being able to handle pressure). I think suddenly becoming worth $100 million could have something to do with it. How much harder do you still want to work? How much do you want to expose yourself to injury.... concussions..... lifetime joint damage.

Bottom line, I think especially when the circumstances are that a QB has had success with a certain HC and then falls apart still under that same HC, that it is mostly on the QB.