Agreed.
I watched all of Gretzky's and Mario's careers, and they were great, above everyone in their day....but players now are so much better overall. The skating, the training, just miles ahead of what it was.
The game is completely different now than in the 70s and 80s. So much faster, and the guys on 3rd and 4th lines now could easily be 2nd line guys (even some first liners) in the olden days.
The amount of skill now is insane.
Not better, just better trained.
The thing you're forgetting is that if you took your players from past years and gave them current diet and training regimes, the gap that you perceive would likely vanish. Not exactly fair to compare generations without equalling the playing field.
Not to mention that if they did take a time warp to back in the day, they'd likely have to be prepared to take a serious beating that they're not even close to being used to today.