I think that's more due to the complexity of the game today. I'm mean, just thinking about the Dline, back then, they rushed against a line that basically put their arms up, grabbed their own jersey, and tried to stay in the way of the defender. Yes, they did gapping and other things, but today's coaches also have to get Dlines to work on defending against the hand punch, leverage against what was called holding back then, etc. etc. It's like anything else. What was cutting edge back then is part of the basic skillset now, and a ton more has been added, which is why there's so many specialty coaches anymore.