The NFL is healthier than it has ever been. It can do no wrong. Every team is making money which is not true of any other sport. They have TV contracts in the tens of BILLIONS. They aren’t going anywhere. They own sports in the US. The NFL isn’t concerned with butts in seats anymore. The owners are. Kind of.
There's nothing that says the NFL has to be popular. At one time, horse racing and boxing "owned" sports in America. Those sports now hardly get more than one or two column inches in the box score pages of the paper; heck, unless it's a Triple Crown race, horse racing isn't even mentioned.
The NFL has some things going for it that other sports don't, mainly that there aren't that many games, so each one can be billed as critically important--a must watch!--and all the games (essentially) are on Sunday, so it is easy for most people to watch.
But, the NFL has a stupid TV policy and it has been slow to embrace the Internet. The tickets are crazy expensive, the players don't make nearly as much as in basketball or baseball (despite much greater injury risk and shorter careers), there are long-term injury risks that aren't present in other sports, and the public is less interested in bloodsport than it used to be.
I'm not saying that the NFL is going to decline in popularity, but I am saying that it could be an issue if it's not careful.