Not going to happen. Had this discussion with many basketball and soccer fans/buddies of mine, feel free to ignore this long-winded opinion of the future of sport in this country.
In order for football to fail, it would have to be self-inflicted wounds. They would have to drastically alter their business model, which is currently the best in all of American sports.
Reasons why NFL is tops, and will stay that way:
1. Every game counts, unlike the NBA, or NHL.
2. Most games only on Sunday. People can count themselves as a hardcore fan, and really only watch the sport once a week. This is the #1 reason, to me. A lot of people, the kind that buy tickets and spend money on their favorite sport, don't have a lot of time. You can have kids, crazy job, and you still only have to get your sheet together once a week, and feel like you aren't missing much. People can watch every single game of their favorite team, which is rare in any other sport.
3. Parity. Every team is a year away from competing. It's proven every single year. Next year some terrible team will make a run, turn out to not be so terrible anymore, and their long term outlook, and reputation around the league, will do a complete 180. Witness the 49ers last season. The worst-to-first storyline is much rarer in the the other sports, and American fans are such that they need to see their teams compete in order to fill the seats.
4. It's the best TV sport in the US. I really don't even think it's debatable. Hockey, it's obvious what a bad TV sport it is when you go to a game live, and see how fast and great it is. Baseball moves even slower than the NFL, and the NBA has much of he game where there is no urgency. Watching the end of a tight game between contenders, and playoff basketball illustrates just how much of the game is mailed in. The NFL, people complain about the break between plays, and talk about how much less action there is than in a soccer match. I believe this is a positive, not a negative. The break between plays in the NFL allows for three things to happen: 1) For the casual fan to get to know the players and game better, 2) for the announcers to build the hype and drama more, and 3) for the replays. As fans, with so many things happening at once, we often miss the best part of the play. A crushing downfield block, a DB making a spectacular play in the ball. One great thing about football, on every play, someone is doing something spectacular. The breaks allow people to catch things they missed, and really lets the fan appreciate what these guys are doing.
5. No competition for TV viewership. This ties into the brilliance of having all games on Sunday, as stated above, but during any other sports' seasons, they are butting heads with another sport. The NFL is so big, MLB wrings their hands in October, being very careful not to have their World Series going against NFL regular season games. The other sports have no problem scheduling games vs. each other. But no one wants any part of autumn Sundays.
Reasons why other sports will not overtake the NFL
NBA: I simply don't see the passion, nationwide, as I do for the NFL. There are massive portions of the country, where people just don't care about the NBA. And how is that going to change? The NBA made it's push with Magic/Larry/Jordan. They may not dip down in popularity, but any big jump in fandom, I don't see where this would come from. Even the cornerstone teams, like Knicks and Celtics, if they have a bad season or two, the fans clear out quick. I happen to think college hoops has a better chance of overtaking the NBA in popularity than I do the NBA passing the NFL. I think NBA players make too much money, care less about basketball than other players care about their sport, and think the fans can tell that. The least-beloved of all pro athletes.
Soccer: This is definitely opinion, but let's start with facts: We have been told for at least 30 years that soccer was the sport of the future in the US. And it is definitely still growing, the TV numbers tell us that. Having said that, those numbers had nowhere else to go than up. They should be going up. The confluence of events that would have to happen for soccer to overtake the NFL are so many and unlikely, I don't see how anyone could possibly feel it was likely:
1 The US would have to have world-class talent in it's pro league. Americans will simply not have the passion for a 2nd rate league. Look at the World Cup. There are many countries where just making the tourney is cause for national holidays. US fans are pissed of they don't make the semis. For soccer to overtake the NFL, fans need to believe they are watching the best brand of the sport. We are a long way from this happening, and I think this is the factor most likely to happen, to give you an idea of how far-fetched this is.
2. Fans will need to develop the same passion as other countries have. I didn't appreciate soccer until I was in Europe, and a lot of the excitement comes from the fanatical devotion, the religious fervor in the stands. Only big-time college football comes close. I believe it is impossible for this to happen within this, or the next, generation. Soccer had an advantage in other countries, it was the first, and in many cases, only sport that they could compete in. The US has too many other sports they compete or outright dominate, in. To a certain extent, a lot of other countries, soccer is ALL they have.
3. The US would have to win a world cup. This is obviously only opinion, but I cannot imagine soccer making a big move in the US without the US taking home the cup, and some stars being born.
4. An American Bird/Magic might be needed. A Michael Jordan of soccer, but with a Wilt Chamberlain as a rival. And they need to be playing in the states, not in the EPL. What are the chances of that happening?