I can see Soccer eclipsing American Football in popularity in my lifetime. Right now, the full-frontal-legal assault against American Football is making it very cost-prohibitive for High Schools to justify having it as an extracurricular activity.
On the other hand, in Soccer you don't need near the equipment (just a ball, a bladder, or the skull of an enemy). Also, soccer games are only 2 hours long....football and baseball games now last 3 hours or more.
If you begin to see poor kids out playing pickup games like they do in Europe and in Central America (like you used to see them play football and baseball and like they do in basketball today), THEN Soccer will be considered America's sport.