It seems that all of those great NFL teams had one thing in common: they all grew from a winning combination of a great GM and/or player personnel director and a great head coach. Bobby Beathard and Joe Gibbs in Washington, Tex Schramm and Tom Landry in Dallas, George Young and Bill Parcells with the Giants, John McVay and Bill Walsh in San Francisco, Dick Haley and Chuck Noll in Pittsburgh, etc. That's because the key to success in the NFL back then was building a roster over time and then riding it for as long as possible.
It’s not parity that does that, it’s poorly managing the cap. Teams that over spend for talent need to split up their line because they can’t afford them. Teams that manage their cap well don’t.