With that being said the TV numbers still really stink and while improving have a long way to go. Also outside of LA the other big metro markets have really not gotten into the MLS. IN particular NYC and Chicago are disappointing with the Pacific NW and mid sized markets with either no or lousy MLB teams being the best supporters with regards to merch.
I expect MLS will be a gate driven league for a long time.
NYC has been a tough sell for two reasons. First, NYRB play in Harrison, NJ in an area that's not that easy to get to. I saw the Crew play at Red Bull Arena a number of years ago and it was easily an hour by train out there from Manhattan. And NYCFC plays in a baseball stadium. Second, it's a city full of "Eurosnobs" - soccer fans who scoff at MLS. It's a problem that hasn't worked itself out the way it has with Mexican fans in California.
Chicago has been an even bigger clusterbuck. They are the first and only club to abandon their stadium to return to their former NFL digs. If I were a billionaire looking to get into MLS, a second club in Chicago might be my move with the Fire apparently being run by a bunch of guys going hard into the melort.
Still, the biggest market failure has to be FC Dallas. They were part of a short-lived wave of teams to do an exurb stadium build, and virtually all of those have proven to be a failure. MLS has since, officially unofficially, pushed for stadiums to be close to the urban center near public transit (they definitely "know their audience"). But there is really no excuse for a team in the Metroplex not selling out 30,000 seats a night, easy. Other than they built the stadium in Frisco.
A new stadium can do wonders if done correctly. Columbus is proof of that. They struggled to get 15,000 into Crew Stadium on a beautiful summer evening. Now they're bringing in 19,000 per. One should expect the other second generation soccer stadiums to be another huge boon for the league.