It's exactly the lateness of the NHL expanding that makes this whole era thing even more confusing to deal with.
The overall talent pool for hockey really wasn't much better in the late 70s or early 80s than it was in the 50s and 60s when the Richards, Howes and Hulls were doing there thing. It's was still mostly Canadians. Only after American and European players started coming into the league in larger numbers did the overall talent pool significantly improve.
Put it this way, I'm not real sure Gretzky had to deal with a lot tougher competition in the 1983-84 season against juggernauts like the Colorado Rockies, Kansas City Scouts, Hartford Whalers and New Jersey Devils (21 teams x 20 players = 420) than Howe, Hull and Richard had to deal with in the 60s against the old original six (120). Would you rather face the best 420 players in 1983-84 or the best 120 players in 1966?
Also think about the level of familiarity. With that many head-to-head games, you aren't going to fool anyone after a while.
Then things change dramatically when eastern Europe opens up and their players start coming over in large numbers. A current team against anyone from prior to say 1990 would probably be a beat down.
Maybe not relevant but I want to add something here. I know nothing about hockey. Zero. I watched the miracle on ice and that’s literally the only hockey game I’ve ever watched from start to finish.
Funny thing is I’ve heard Gretzky’s name and Lemieux throughout my life. The other guys you’ve all mentioned...never heard of them.
The only Era-related comment needed for me is number of teams. 6-12 vs 24+ teams is a huge difference. Basically anything pre 20 teams doesn't really do it for me. Obviously skill has gone up over time, making it harder to be the best as the gap narrows, but the lack of teams in the old times means that you could basically just out coin-flip others to be the most winning.