Here’s what I have learned in the divisional games… The HC only matters a little bit.

Nonsense. HC is the most important position on the field and the sideline. This has been proven for ages across all sports.
I think the OP is saying that the coach can only take you so far if you don't have an elite QB. Just look at Pitt, they can't get past the first round despite the greatness of Tomlin. I think we are starting to forget how great Drew in his prime was.