This is not a defence of KM or any other specific OC for that matter, but just some things to consider when evaluating an NFL offence.
In between the 20s your scheme and your OC can make a big difference dialing up plays, misdirection, etc to win. There is lots of real estate and the pressure is on the defence to cover a wider territory.
NFL OCs with longitudinal success year after year are the OCs who can scheme and dial up winning plays inside the red zone, and can consistently get their teams out of the pinch when their backed up inside their own 20 or 10. Those two zones are where the big boys come to play.
They are also the areas where the effectiveness of the coach and OC and scheme becomes less important, and things like physical talent, and execution are the primary reasons you win on a consistent basis.
Players who have quickness, have height, have great hands, superior leg drive and can anticipate and see opportunity do well here. Combine that with great practice and repetition to nail down execution and there is where the teams that do well here reflect on the OC and his schemes.
In the red zones (yours and theirs) you rely much more on your players and your position coaches to display talent and execution.