Sure; that fact is a legitimate concern.
However, when McVay was with Washington, it's not like they were runaway train of scoring.
So the question is how much of something like that is hung on the DC, how much on the HC, how much on the players themselves?

Look. It's no different than the acquisition of Stafford. It's not like he comes from a winning record. But how much of that is on him, how much on his OC, HC, or the players on the team?

We'll know the answer to all this about this time next year. And despite all the kudos, or the hand wringing, both are just speculation and opinions.