When the money for US professional soccer is comparable to football and basketball. You say you don't want to hear the lame excuse that the best athletes play other sports, but it's still true. If you're a top athlete you learn how to catch footballs because that's where the money is.
As far as the money goes, I think it'll get there but it'll take a lot of time. As you mentioned, soccer has grown in popularity for 25 or so years. That means that the older generations with money still don't get the sport for the most part. As the generations that grew up playing and watching soccer continue to support MLS and such, sponsors and TV revenue will follow.
I agree that if MLS offered even better salaries (and the league is already pretty good statistically and last I saw around 15th league in salary average for players worldwide - which is skewed by the massive salaries some of the Designated Players receive) then we'd probably have even a little deeper USMNT player pool. If it was on par with NBA, then it would help.
But, I don't buy the "if the best athletes played soccer we'd be better theory." I think we would need a cultural change too. Just look at some of the best soccer teams in the world lately. Spain dominated from 2008 to 2013 or so. They played a constant ticky-tack game with great passing, vision, and touch on the ball. Their players didn't have extraordinary quickness, height or leaping ability. They have very very good physical abilities, but they weren't freaks in some way like NBA guys can be (hyper tall with long arms). So often I see it bandied about in the U.S. that if Lebron or Kobe grew up playing soccer, they could be pros. I just don't buy that. Those European countries have a lot of big, talented athletes as well, but they aren't playing soccer. Smaller feet help in soccer, and the big feet of super basketball athletes like Kobe, Lebron, etc. would not help them play soccer. Physical attributes which are assets in basketball aren't in soccer.
Anyway, I would just argue that the U.S. is already "good" at soccer. We're probably going to qualify for our 8th successive World Cup in a couple months (albeit out of a weaker qualifying region). There are only 9 countries total that have made the World Cup every year since 1990 (Brazil, U.S., Argentina, Germany, England, France, Spain, South Korea, and Italy). The U.S. average since the inception of the FIFA world rankings is 20 and they have gone between 7 and 35. So, being above the vast majority of countries, I'd say the U.S. is already "good." We just aren't one of the very few handful of soccer elite.
If we can just develop / uncover a couple more like Pulisic, we will have the elite quality throughout our 11. We're just a few short because it isn't our national past time, but I don't think having players like Kevin Durant, or Cam Newton, or Justin Gatlin (track) playing exclusively soccer would have made them world class soccer players. That's a simple way of thinking and as someone who played soccer, basketball, football and baseball growing up, they require different skills and abilities and while Lebron would have been able to likely be a dominant header on crosses into the box if he was a forward, he would likely have been a liability on many other areas of the game.
This argument always provides good "barbershop" banter though.
