Where they disappearing to? Baseball?
No, baseball’s fan base is way older than football’s.
The millennials are more into participation sports. Running is huge. CrossFit is big as well.
The major sports like baseball and football became huge in an era when there were only three major networks. So people really didn’t have many options. The Internet broke things wide open. Now, niche sports can get a lot more visibility. You’d think this would be a good thing for bodybuilding but the current state of the sport is so offputting that it hasn’t capitalized it all.