Well, Americans are improving their football skills and that's a fact. But, I just can't see them winning a World Cup. I am not saying it will never happen, but it would be very weird. Mainly because US never seemed to be interested in football as much, neither when they hosted 1994 WC. For me, and sorry if I am wrong, it is all about being the best. They are a very competitive country in everything they do and it has to be also in football.
As I said earlier, you don't wake up and say "I am going to be the best football player on Earth because I am American and I can do it if I want to". Maybe, from here to the next 40 years, people there have changed their minds and realise football is not about being the best and win everything.
Anyway, I always tried to figure out why Americans are not a "football people". Do they find it boring or something?