Think of it this way: If 18 teams run a scheme designed to create an above-average offense – not every one will rank above-average statistically in a 32-team league.
It comes down, then, to execution. A scheme is only as good as the players asked to run it and the coaches tasked with teaching it, people in and around the NFL say.
I agree with that second part 100%, and have been saying the same all offseason. Scheme can only do so much.
No OL? Forget about it.
Good OL but bad QB? Hopefully the run can keep us in games, but it's not a formula for success.
Good OL and good QB but bad OC? Now we've finally reached the point of being sure that the bad offense is the fault of the play caller. If the OL and/or QB are bad, there's not much an OC can do to salvage the offense.
Payton went on a streak of 15 straight seasons as a top 12 scoring offense (with 10 finishes in the top 5), but quickly fell back to earth with the 19th best offense after Brees was gone. He schemed us away from being among the worst, but he wasn't going to replicate his success once Brees was gone. The Saints were actually graded with the #1 pass blocking line by PFF that year, but we finished dead last in passing yards.