The problem is that four other league already take up a huge part of the media and sponsorship landscape (NFL, NBA, NHL, MLB), plus there are some big college leagues as well (especially in football, right?), so it's just hard for the MLS to grow. It would have to be at the cost of another sport. MLB seemed set for that, with dwindling everything; but they may have stopped the rot, plus I'm not sure MLS could take MLB's place, given the deep difference between their sports.
But who knows. It should certainly help that the MLS keeps expanding, cause more teams means coverage in more areas and more chances of attracting their local soccer viewership (which might now rather watch the EPL). I don't think the quality of players matters much for that: it's not like the EPL is bigger than the Eredivisie in the Netherlands, despite the gulf in quality. And maybe it will help that soccer is relatively popular among girls and women, cause that's a demography that all the other leagues I listed don't cater to quite as much.
So it all remains to be seen; and in the meantime, at least the MLS continues to be a healthy and growing operation. It's not like it's barely hanging on, like it pretty much was in the old days of the New York Cosmos.
Salary cap (noble idea), no relegation and drafts mean it's more or less impossible to relate to the league.
That's all standard in North American professional sports, so that won't stop things. Well, maybe outside North America, but then I don't suppose anyone is thinking the MLS will overtake European leagues in global popularity (let alone in Europe).