They're throwing less because they don't get the time to throw, not because the WR's aren't getting open. Go and listen to the Coach talk about this himself at the official site, multimedia section. Sure, it happens once in a while that none of the receivers get open, or JP misses seeing an open guy, but is certainly not the crux of the matter. When you run a 5 step drop passing play, and cannot get past 2 and 1/2 seconds before the qb gets contacted, it's the line. Jp's completion % hasn't really wavered whether throwing 13 passes or 30 passes.
Also, blaming the offense's problems mostly on the line is not "mindless". It's reality. And it has been a problem here since the mid nineties. At times, they've been able to run block pretty well, but the pass protection has been consistently putrid throughout those years, with the only spell of success coming from a Qb who's whole game was based on improvising and running in circles.