I don't really have a dog in this fight, but I'm thinking a lot is getting lost in translation here.
A lot of people are looking at this thesis the way OP puts it which is "If the U.S put it's best athletes on the soccer field tomorrow, we would kill everyone immediately" which obviously isn't true.
But if we handled "soccer" the way other countries do, then yes the United States would be elite. But there are a lot of IF'S here. International soccer is ridiculously competitive and has been for a hundred years. This means that as far as coaching and teaching kids the game goes, the U.S is WAAYY behind. Even if U.S has the best athletes they might not have the best coaching. It would take decades for U.S to become a soccer factory. They would have sent their best players overseas to play as youths like they do with Hockey.
Its definitely possible, but a LOT would have to change. It would take years but its possible. You can't win with sheer athleticism alone, but to put it into perspective, if you took an entire generation of would-be American slot receivers, running backs, point guards, and sent them overseas as toddlers to grow up playing futbol, and sent them back to the U.S, you would definitely see a difference in the country's ability to compete internationally.
If you're playing university soccer its too late for you.
By 18 you should be playing for a club.
That's the problem we have in america. The traditional youth league, to high school, to college, to pro, american path to becoming a professional athlete doesn't work for soccer, and the rest of the world doesn't follow that path at all.
In america we still like to think of our athletes as students first (or act like it). The rest of the world doesn't give a fukk if their soccer players can't add 2 + 2 as long as they turn into world class soccer players.
^^ this pretty much nails it. Americans would have to go about soccer like how it goes about baseball. If you're good, you get a contract, no matter how old you are. Cream rises to the top, no school required. Just like how Griffey and Bryce Harper were already crushing shyt in the MLB as teenagers, Messi and Neymar or whoever were crushing shyt in their respective leagues as teenagers.