While I totally agree with the contention that today's hockey has too much stick work, I think the drop in scoring is not related to that. I put that down to 2 factors: 1) advent of the trap (thank you 1990's New Jersey Devils) and 2) changes to goaltending style and equipment. 
With the exception of Mike Palmateer (who had no style at all, beyond "salmon out of water") the rule of the day was "stand up goalie."  Watch Ken Dryden play goal. I'm not sure his pants ever touched the ice in some games. That changed with Dominek Hasek and the advent of the butterfly style. Add in near-lacrosse sized padding and there's a lot less open net to shoot at.
As evidence, shots on goal per game have not really changed. I haven't bothered to check, but an average of around 30 shots on goal per game was the norm in the 70s and 80s, just as it is today. If slashing, x-checking and general obsruction was truly responsible for lower scoring, that should be reflected in lower shot totals.