The top-ten scorers in 52-53 consisted of 6 Hall of Famers (including Howe). The top-ten scorers in 65-66 consisted of 7 (including Hull). That's not the huge difference you're making it out to be.
Averaging the top 3rd to 12th Canadian-born forwards in points-per-game (minimum of 25 games played, from
), the average was 0.763 points per game in 52-53 while it was 0.946 in 65-66. It was simply a much more physical and defensive-minded era in the 50's. Certainly, you wouldn't attribute the return of players after WWII as watering down the league... but scoring dropped from 8+ goals per game in 43-44 to 4.8 during Howe's peak (and relaxed to 6.08 goals per game in 65-66). (Going the other direction, league scoring rose as more and more players left to fight overseas.)
Adjusted to 2015-16 scoring levels using the 3rd to 12th Canadian forwards as the seasonal benchmark, here are the league leaders until 1967-68 (right before the Orr/Esposito era):
Even longevity is an era effect. Before Howe came along, only two players had managed to win the retro Art Ross in back-to-back years (Conacher and Schriner). Injuries (thanks to general viciousness and recklessness) plagued the league. Morenz, in his all-time great season, managed to suffer injuries to his kidney (shoddy construction), head (twice, with the second time being a cross-check that split open the back of his head), and fingers (broken). He missed only one game, but that undoubtedly had long-term consequences. Advances in training and medicine over the years have definitely allowed players to extend their careers.