I watch football because I like it, not because it's popular.
Really don't care either way. Gridiron football is gaining worldwide popularity, and soccer has been gaining US popularity. Seems fine to me. As to who over-takes what, I don't care. Hell, most Americans don't care, as football isn't the most popular American sport in the world. Basketball and Baseball are more popular than Football, worldwide.
Soccer is something like 3.5 billion fans worldwide. Cricket is close to 3 billion, and basketball is not far behind with close to 3 billion as well. I think Football is around 500 million. So yeah, sure. It's entirely possible to see soccer over-take football in the US at some point. Maybe in 20 years, maybe in 100, who knows.