Assume for a moment that what you posted above is plausible - you still have a major obstacle to overcome. Ray Bourque maintained the pace of the game during his first season, 1979 -80 and during his last season more than twenty years later. This is true for all the players who played 15 -20 + seasons, consecutive or not, at various points in NHL history. Yet you have failed to explain within the context of your hypothesis why this is so. So step - up and do so. Consider Chris Chelios,Gordie Howe, Jean Beliveau or various goalies, Patrick Roy, Jacques, Plante, Dominik Hasek etc.
Kindly explain why a 51 year old Gordie Howe was more productive than an 18 year old Gordie Howe:
http://www.hockey-reference.com/players/h/howego01.html
Basically your hypothesis does not hold or in the alternative, despite all the advances that you admit, the collective skills of the other 1980 era NHL players had collectively regressed compared to the late 1940's era.
Conversely if your point about the level of competition holds then you have another issue - equipment. Take car racing. Be it NASCAR, Indy, F1 the actual car is better today than it was 10 - 20 -30 years ago. On the other hand the great drivers will span the eras and win whereas the cars would not.Effectively great hockey players with greater equipment, technology, training, coaching tend to adapt favourably to the various positive changes with enhanced performances.