This thread pops up on a regular basis.
Will soccer become the most popular team sport in the US measured by organized participation? Yes, I have no doubt that it will and it will happen within 20 years.
Will soccer become the most popular spectator sport measured in TV viewership? That's more questionable. There's enormous amounts of money in football and baseball which have a stranglehold on the sports media. And those two major sports basically have a non-competition pact in that their seasons barely overlap.
Soccer will be challenged to break into that company for a number of reasons:
1. The best leagues are in Europe and that will never change
2. For the MLS to at least become a second-tier league it would need to play a longer season which would really put it at odds with the big sports
3. MLS can't rely on drafting talent out of NCAA - the best youth players are signed by top Euro clubs from age 15-16 and only those who fail would consider the MLS before retirement age
4. The stadium experience is important in soccer and the vast majority of fans will not be even in driving distance of a league team until a proper multi-tier league system is established
5. Not enough commercial breaks in a match to attract the big networks
To those who say it's boring to watch, that's not a valid argument. Sure, the current generations of adult Americans have been indoctrinated to watch sports as a series of discrete plays with measured outcomes, and soccer doesn't fit that. But today's younger generation and future generations will not be that way.