I think you're right in that the QB is the limiting factor for any offense. Think of how Aaron Rodgers, Drew Brees, and Tom Brady function with regard to pressure -- specifically how they evade it and often hit receivers downfield for big gains -- and apply that to how a defensive coordinator has to adjust with regard to the assault he places on the offensive line.
Now apply that to lesser QBs, and imagine the freedom that gives defensive coordinators to apply pressure, and how that makes opposing offensive lines look.
Then combine that with often being down on the scoreboard and being forced to pass, which is often the case with lesser QBs, and imagine now what defensive coordinators have the freedom to do.
In the end you have an offensive line that can't help but look like swiss cheese. The defensive talent throughout the league, combined with defensive coordinators' ability to let it run rampant under the above circumstances, would overwhelm just about any offensive line.