If MLS tries to expand to every major market, it's going to have a terrible product. A 30+ team league is going to be a joke. Good American players will want no part of it. What you'll get is a league starting way too many guys who should be playing a tier down. Take a few low quality players and start them on Barcelona and you'll get an unholy mess. Bad players are pernicious in this sport. Deportivo Saprissa is going to bully MLS sides at will. Mexican teams will go back to annihilating them with B sides. This sport does not exist in a national bubble.
My local MLS team has chased away fans for years. Bad teams, bad stadium, bad marketing. They could win MLS Cup and it would barely register in Boston (which is a city filled with soccer fans). The Premier League is on NBC. Kids will be watching World Cup games and Champions League in ever increasing numbers. MLS may never even be the primary soccer outlet for most U.S. fans. If it really wants to draw a national audience and woo the next generation, it had better put on dynamite games played at an extremely high skill level. We get Cristiano Ronaldo highlights on Sports Center. MLS needs to worry about measuring up.
I agree that's the way to grow it, but if the end goal is to have a team (or two in some cases) in every major market then they need to think about creating a real pyramid.