This won't happen. At least not overnight... or even in 20 years. The best players will always want to play for the (historically) best clubs. It will take a long time for (if) the MLS to become a top-tier league. Nowadays, geographically, much of the world's stars play in Europe. Even if they migrated to the USA over time, how would it affect the game (UEFA Champs League, etc)? It would take time and, ultimately, MONEY. Lots of it.
This is the reason why Klinsmann denied the job to coach the US team years ago. He wanted to entirely revamp the US youth system, taking the talent out of the country into more competitive leagues. Mr. Gulati does not (and will probably never) agree with that sentiment. It is a shame that the USSF is so ethnocentric... we need some global influence to consistently yield a decent threat in big tournaments. Until then, we'll always be the little engine that could (and didn't
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Don't get me wrong, I'd love for the MLS to become a big league... but I just don't see it happening until I'm at least middle-aged. Look at what Man City is trying to do with their unlimited funds; not only is the club vastly inflating the transfer market, it has yet to win anything. Only time will tell how influential money is as opposed to history, desire, and smart, shrewd managerial decisions.