I think it will be different this time around. In past soccer "expansion" efforts in the US, the growth was too much/too fast and you had some teams that just spent money on aging superstars, and then the quality of play suffered.
This MLS effort over the past 10-12 years has been very disciplined in expansion, you are seeing purpose-built stadiums, and you have a pretty good product on the field. There has been some good parity, you can see teams going from the top of the table to the bottom and back again, and I don't think any city is saddled with "the Detroit Lions" of soccer.
It comes down to pay. The US market is huge. If the European league stars can find comparable money here (particularly with endorsements), you will start to see more and more players who choose to play here DURING THEIR PRIME YEARS.
But the team salaries will need to rise. If these owners are sitting on more-and-more-valuable franchises, they need to decide to plow some of that value back into salaries.