HCs are absolutely a potential reason for a QBs decline. I'll list out the reasons but first on Wentz.
It's a little different situation because the OC is a huge part in a QBs success and in Wentz' case he lost Reich. In Goff's case, McVay has pretty much always been the game day play caller with some others as "OC". If we see Wentz come back to form under Reich, then HC was a part of the problem for reasons below but very different circumstance from Goff/McVay.
So how can a HC cause a regression in a QB?
1.
Communication. HC has to see the gaps and ensure an understandable plan is in place with the entire team on board and rowing the same direction.
2.
Personnel Decisions. Not addressing short comings at any of the offensive positions (OL, RB, WR). HC needs to make sure the roster to succeed is out there.
3.
Culture. HC sets the culture. If it is one that is too tense, or too loose that can cause regression. Are players treated fairly? Are they given individual attention?
4.
Style and Play Calling. One example is ultra aggressive but risky. Shanahan runs a lot of naked protection which gives Garoppolo more targets but also less protection. McVay is the opposite. Injuries can cause a QB to regress. Getting hit too much a la Russ Wilson. Some guys just get injured more but the HC can account for this in style.
In the case of Goff and McVay, my view is that McVay was magic for Goff at first. McVay got Goff to play at a high level with an innovative wide spread attack that destroyed unprepared defenses. Eventually the best defenses learned to put 6 guys up at the LOS and that took away the wide and gave additional blitzing packages. McVay hasn't been able to counter and the offense has regressed. Stafford may help some by handling the blitz better, but that exact offense may never be great again.
Now to Jared in particular. My guess is McVay has been harping on Goff to stop turning the ball over. He is relentless in his focus on it. Goff has now become more trigger shy and hesitant. He is turning the ball over even more and not taking many shots. He is having to account for all kinds of blitzers in this offense he is not comfortable doing. He'd be better off going back to the shotgun and spread offense where he has more targets and less blitzers.
To me these are the plausible explanations of Goff's regression (and it is likely a combination of these):
1. Goff is over-coached. Let the dude play and not be thinking about a million ways he could screw up.
2. Personal have changed. Less good targets and more guys staying in to block. OL inconsistent.
3. Offense philosophy to keep more guys back and protecting then delayed leak out is making for harder reads on the QB
4. Goff Wealth/GF have made him less hungry
5. QB/HC relationship has soured and affected his play
6. Goff has gotten hit enough that he is finally feeling his mortality and has become gun shy
7. Goff has some kind of shoulder injury affecting his arm and deep ball that is undisclosed
In summary the HC is Top 3 in reasons a QB regresses or thrives.
Side Note
The Lions fans are doing a lot of interesting comparisons of Stafford of 2013 to Goff of 20. Caldwell came in and "fixed" Stafford in their eyes. He showed him how to play the position at a higher level to where he is now. Goff has better stats even with 19/20 than Stafford through 2013.
