People have been saying this forever, and it's a ridiculous fiction, largely premised on the ideas that (1) there is such thing as generic "athleticism"
Look into the sports gene, or just open your eyes and look at different body types.
and (2) if the U.S. isn't the best at something, it's because we don't want to be. (There is some element of truth to the latter, though world soccer has proven remarkable in the degree to which the tiny countries are able to compete.)
Yes, there is some truth to this.
Nobody decides to play soccer because they're not good enough or athletic enough for another sport. They play because people in their family and community play--it's what they're exposed to.
True in most cases, but despite your apparent disbelief in genetics, this just means that less athletic families produce less athletic kids.
By the way, youth baseball and football, in particular, are totally withering away,
"Totally withering away" is an exaggeration. Black kids haven't been playing much baseball, and white kids less and less as well. (See the single mom theory - for both white and black).
and no kid EVER chooses track over a team sport if they're good enough to be a team sport star.
I wouldn't say NO kid, but you have a point to some degree.