I think this topic proves that no one has a clear answer and just wants to blame it on whatever they value as an opinion. This is a Colts forum, so it's almost predictable that it will get blamed on Peyton Manning's retirement. This is the only place I see where the Brady/Manning games were taken seriously...I don't think most fair weather fans cared at all, it was just another game for us. If you're going that route, throw in Ray Lewis' and several other retired player's names...The league does not revolve around 2 QB's in a little rivalry that lost it's mojo when Manning went to Denver.
It's kinda like how people love to say "gee, pop music today is terrible! What happened?" There is an incredibly amount of bias to this that often shows the age of people who say it. Ages 35 and up often let nostalgia cloud their judgement. Like music for instance, it's easy to say that when you're just remembering the classics and the good stuff that stood the test of time and aged well. Every decade had horrible music, the songs that everyone forgets for good reason. I feel this is the same in sports and especially NFL. It's easy to say the old days had more dominant teams when you're only remember teams like the 49ers and Cowboys of the 90's. There was also a lot of bad teams in those same years. Same for quarterback play.
No sport in American history has reigned supreme forever. Boxing used to be the biggest household sport for the longest time. The heavy weight title was considered a crowning achievement, and that sport eventually died off to irrelevancy. MLB has the steroid era, NBA it's slow decline after the 90's....The NFL is going to decline like everything else did.
The true answer probably lies in CTE research and everything coming to light with concussions that we previously did not know beforehand. I think a lot of consumers and fans view the NFL as being more dishonest and more corporate and about the rich owners. Many of them see the NFL as they don't really care about their players like they do other professional sports...Roger Goodell is universally hated of all major sport commissioners and for good reason; he's a laughing stock and a phony. It also don't help that the fans have to sit there and witness Goodell and this league who clearly care far more about money than them with moving teams and re-locating them and the new stadium costs. Just look at how empty the 49ers new stadium is every game, or how the people in San Diego had their beloved team torn from them thanks to a sleazeball owner and a commissioner that couldn't just put his foot down and say no. Not every fan is going to keep watching this sport after they get royally screwed over and their beloved team ripped from their city and areas.
The NFL gets what it deserves. They wanted to keep Goodell around, and wanted to bring two teams to L.A. so bad while ripping them from their fans (not counting the Raiders move to Vegas, that's going to be bad too), and add in how dishonest they've been about the concussion stuff while former players suffer.
Most NFL fans are also "fair weather". When the team is doing poorly, they're not going to bother spending outrageous amounts of money to go to games and be surrounded by fans of the road team. Even a mediocre team, there is no point in wasting your money if they aren't having a winning season. But if the team starts winning and gets it together, all those fans are going to come back.