I think everyone is over complicating this. Its pretty simple.
Josh Allen is not as effective this season as in recent seasons. This is not open for debate.
Why Allen is not as effective this season is open for debate. The following is my opinion
McDermott is a system coach and wants players that conform to the system. The alternative is an adaptive coach who modifies his systems to optimize the talents of the players. Both types can succeed. I do prefer the second type. I believe Bellichek and Reid both are/were both more of the adaptive types.
Allen, like most high end athletes perform best when in the proper rythym. In football one of the primary goals of the defense is to get the QB out of their natural rythm. In games where Tom Brady struggled, it was almost always because the defense got him out of his rythym.
This season the Bills offensive scheme takes Allen out of his rythym, which saves the defense the trouble.
Every year with the Bills it seems like there is a new grand experiment from the coaches. This year's experiment is: can we change Josh Allen's natural rythym to conform to something that is not natural to him, and get equal production. So far the experiment is failing.
The problems with the WR corp are real, but they are secondary to to the problems this experiment is creating.
The solution from my perspective is that In the first or second series of each game, the defense needs to be reminded that Josh is a running threat on passing downs, when Mr. Cook is having his well deserved third down rests. This will put Josh back in his natural rythym, and it will make the defenses account for that the rest of the game. This one change would have a profound impact.