Being an American and exposed to all sports and having played many (soccer, baseball, & golf primarily), I've known many good soccer players who were good athletes and could compete in other sports but I've rarely met athletes who excelled in their respective sports and made the transition to soccer with the same ease.
I get the gist of your post and it does make sense to an extent.
If I'm not mistaken, Ajax's youth system would identify players at an early age and categorize them by speed. Regardless of what positions they had previously played or been exposed to, the fastest players were groomed as strikers, the next speediest group would be groomed as midfielders, and then the last group as defenders.
Obviously there were exceptions but generally, they'd try to get as much speed up front.
There's no doubt in my mind that if some of America's finest were exposed to soccer at an earlier age and stuck with it, that we'd have some great ones. We're just now starting to see that happen and now with the advent of a professional league here. Younger players have something they can strive for.
But with the large contracts going to gridiron football, baseball, and basketball players, most of America's top athletes will attempt to make the grade in those sports as opposed soccer.