Soccer is not going to touch American football for a long time, if ever. There are quite a few reasons for this but the same reason people jump on the USMNT bandwagon is similar to why some will fight tooth and nail to hold on to the NFL, NBA, and MLB... they are inherently American creations. We love America thus when we do something better than the rest of the world we support it like crazy. There is 0 foreign competition for football (lack of funds and interest in other countries), little for Basketball ($$$$), some for baseball (Japan, etc), and more for hockey. The popularity of sports in America seem to be directly tied to A) are we the best at it? B) are there any other/better leagues as options? The popularity of sports in the US directly coincides with how much competition there is for viewership of said sport ie NFL>NBA>MLB>NHL. NHL growth is limited due to many other high level leagues as well as limited area's of the world where people could even consider playing it. MLB is a slowly dying entity as it goes the way of radio broadcasts and baseball cards. People will argue that because they love the sport but their are foreign leagues that will hinder international growth of the MLB as well as a fair amount of the civilized world that couldn't care less about baseball because they have cricket. NBA essentially is the EPL as it pertains to pulling most of the worlds top players but without the far reaching potential of soccer. I have a friend that grew up in Zambia and never heard of basketball until he came to the US but played soccer all the time.
NFL are modern day gladiators which is why the viewership is so high and will remain so. NFL and NBA share the same wow factor that soccer, baseball, and hockey will never have in that most of us can run, pass, shoot, tackle, etc BUT what most can't do is be 6'4 +220+ lbs with a 40 inch vertical and run a 4.5 40. This is why NFL and NBA draw so well, these guys are monsters that can bench bress a house and out run a cheetah. Soccer, baseball, and hockey players look like us vs being giants. Anyone that has tried to replicate what players in any of those 3 sports do on a professional level know it takes an absurd amount of skill but because the players of said sports look like everyday people a lot of casual observers aren't initially drawn in or assume it's not that difficult if a normal looking human can do it.
IMHO by the end of THIS decade viewership of sports in this country will be in order football>basketball>soccer>baseball>hockey. I'm sure that will p*ss on a few baseball fans cornflakes but if you're not a fan of maybe 5 teams the support for an MLB team is regional at best. Think about how many fewer baseball team hats/gear you see walking around everyday. How many kids do you see in the back yard throwing baseballs around? Soccer is 10x more accessible now than it was 10 years ago and I think that plays a big part in the rising wave of popularity (thank Al Gore for inventing the internet!) while major networks try and cram baseball down our throats and fewer people care about it everyday. The only way Soccer will even come close to football or basketball in the US is if our domestic league gets a lot better and the NFL and NBA complete up and alienate college fans by trying to turn it into a "minor league" rather than a feeder system. Stay tuned for that if a few more college players feel like they're too big for their britches.