This is a great post overall, well done. Your last paragraph though I'd take small issue with.Comparing eras is so difficult. For me no question if you put a guy like Mc David head to head against Gretzky, Mc David would torch him, by virtue of his skating, and edgework. Skating today is so much better, I mean there were still players in the 70s and 80s who couldn't skate backwards. So it's a tough comparision. The league today is more talented than ever, there's a recognition that goaltending is the key to success too.
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I agree 100% McDavid would torch Gretzky, but we have to consider the era there as well. Gretzky's coaching probably did consist mostly of his dad and his off-season regime would have been so very different. Put Gretzky in a development program(s) like McDavid has experienced and I think we have an entirely different conversation going on. The improvements in skill development, nutrition, strength training have been astronomical.
This is the biggest difference between era's to me (not to take anything away from the excellent goaltending discussion), regardless of sport. I remember my parents buying furniture from Kenny Ploen in the offseason for heaven's sake. Kevin McHale's offseason regime consisted of fishing and drinking beer on Minnesota lakes (basketball player for Celtics for those that aren't aware) yet he was a pretty dominant power forward in his era that had Bird, Magic, Kareem and others. The Hull brothers would likely be considered as raging alcoholics by today's standards and the sheer number of players that used to smoke a couple of packs a day boggles the mind. I'm not convinced Bjorn Borg could take a set off Federer if you don't factor in eras and regimes.
Today's athletes are by far the best of the bunch, but if we're ever to compare eras we have to consider a myriad of intangibles.