Good analysis! Happens all the time in all the sports.The McVay "magic" is gone. He's a Coach now. Sometimes good, sometimes not so good. Took the league by storm and nobody knew how to counter him for a while. As a play caller, he's clearly not the best. Shanahan in SF, and I suspect Johnson in Chicago are better.
New guy comes in with fresh ideas and is almost immediately successful. Martz was a real controversial HC, but he created an offensive system that few could stop. But, these supernovas eventually burn out and get beat.
It happens... and it is very possible it's happening to McVay.