On this point, here's a long but well written take on the future of MLS that touches on the idea of a permanent "second league" in sub-2m metro areas (with no talk of promotion / relegation):
There are many cities like Indianapolis and San Antonio that a second tier team could be sustainable in, without necessarily being a fit for MLS, while smaller crowds in smaller cities could also work sustainably at the third tier. Oddly enough, that’s perversely shown by the stubborn survival of the stepchild of American pro soccer in places MLS has yet to land. Milwaukee and Baltimore have since the 1980s both been supporting professional soccer teams: just ones that happen to be playing indoors. The fact is the appeal of indoor is waning fast: if the debate now is about getting the Manchester United fan to support MLS, getting him to support MISL is far-further-fetched (I say that sadly, as a former staffer in the MISL and a fan of the fast-paced game on its own terms).
EPL or La Liga fans in Milwaukee or Syracuse or Wichita who grew up playing soccer are likely waiting for an outdoor professional soccer team to emerge that they see as a serious proposition to support. You won’t get all of them. But you can get enough of them if you give them a chance to build on their own organic passion for the game.
Those fans want a team to support without a silly name and logo, playing roughly the same game they have played and see on television every weekend, a club connected to their community that they are proud to wave a scarf and bang a drum and build a tifo display for. By doing that, their collective presence – often coalescing in supporters groups – helps make it an exciting proposition for kids and families to fill the rest of the stands.
There are thousands of these soccer fans in American and Canadian cities and some of them – like the Brickyard Battalion did in Indianapolis or the Crocketteers did in San Antonio – got off their couches and helped bring pro soccer to their cities without waiting for MLS to wave a magic wand. In Baltimore, they’re already embracing the pretty damn cool PDL Bohemians, just as Detroit’s fans are embracing their NPSL team. If amateur soccer can generate that interest, pro soccer would drive it to another level – if each league, under the oversight of US Soccer, remains focused on being stable and sustainable, tapping carefully into the right places to grow the game over the next decade.
The article touches on a great deal more points than just this and is worth a read, IMO.