Within one season more powerplays would increase 5 on 5 goals because players would clutch and grab a whole lot less. Eventually power plays would be reduced just by players being more disciplined. Problem is the league never makes refs do their job.
Again, I watch a ton of hockey, not just my team, and I fail to see uncalled clutching and grabbing as a major problem. Are the referees inconsistent at times? Yes. But I don't think clutching and grabbing is currently having a major effect on 5 on 5 play. Like I said before, go back and watch pre-lockout hockey. There it was having an indisputable effect on players ability to generate offense at even strength. The two-line pass rule didn't help either.
What you are describing has already happened. After the lock-out, there was an insane amount of power plays due to players having not yet adjusted to the new rules. Eventually the "new NHL" forced players like Derian Hatcher or other big, slow defensemen out of the league. Once these guys were fazed out and players got used to the new rules, there was less clutching and grabbing and fewer power plays. Fewer power plays led to less scoring.
I attribute the decline in 5 on 5 scoring to parity (there are fewer terrible teams and fewer unbelievably stacked teams like 80's Oilers or 90's Penguins, Avs, Red Wings), improved defensive systems and lack of teams willing to play run and gun hockey, emphasis on shot blocking and bigger goalies / goalie equipment.
I think the Dallas Stars are a good example of how much coaching can influence scoring. They trade chances a lot, and play pretty wide open hockey. As a result, they are first in the league in ES goals per game. They are also 26th in the league in ES GA. Part of this could be attributed to having received some substandard goaltending, but it could also mean that they give up high quality chances. The point of this is that they play old school run and gun hockey, have 2 very dangerous lines, get significant depth scoring from guys like Roussel and Garbutt and get a lot of offensive contributions from defensemen like Goligoski, Daley and Klingberg. Because they are one of the few teams that puts offense above defence or even puck possession (I'm looking at you Minnesota), they are proof of the big role coaching has in the way the game is played today.