The lack of powerplays is definitely a very real concern. I don't think it attributes to the significant reduction in production from the very top players, though. We're talking 20 points plus - a small increase in powerplays would only put a small dent in that differential. This is a legitimate outlier season.
Final NHL scoring leader positions 2-10:
14-15 season, 86, 84, 81, 81, 78, 77, 76, 76, 73
13-14 season, 86, 86, 84, 82, 80, 80, 79, 79, 79
That's a huge difference to you? The only outlier was Crosby blowing the league out the water last season. The PP conversion actually went up this season from 17.89% to 18.66%, but the PP opportunities went from 3.27 last season to 3.06 this season. This season's slight reduction can be attributed almost 100% to the lack of power plays and the corresponding dead puck 5on5 play.