I can't speak for anyone else, but for me, I tend to believe what I see. The Celtics with Bird, McHale, the Chief, and DJ were as good or better as any team in the NBA today. Same is true of the Lakers with Magic, Kareem, and Worthy. They were more fundamentally sound. They played better team ball. And they had multiple Hall of Fame players on their teams. (I hate the Celtics.)
Regarding the NFL, it's not just about me being a Niner fan. I absolutely hate the Cowboys. Despite my hatred, I can see that Aikman, Smith, Irvin, Harper, Novacek, and the O-line is/was one of the greatest teams in NFL history. Better than most of what we see today.