You can't directly compare soccer in England to soccer in America. It's very tempting to say soccer will work the same everywhere, but it's much more apt to compare soccer in America to other sports leagues in America.
When did Garber say 30 is too many? I'm surprised to hear him say that.
But honestly, he might be right. Quality expansion candidates are going to fall in number at the same time that the expansion criteria will get more stringent (rising franchise values will increase the expansion fee rapidly!). Expanding from 20 to 24 was relatively easy and necessary for the league. Going from 24 to 28 will be a little tougher, and a little bit less necessary. At 28, we have pretty successfully saturated the country with teams that cater to just about every region or sub-region. TV contracts will be high. The marginal benefit from going from 28 to 32 is much lower, and could take a decade+ to find the right markets and investors.
I mean, tell me what market is available, after 39 other teams are already in existence, that is worth a $1 billion expansion fee??? It's just not going to happen, there are too many competing sports. And existing owners aren't going to devalue their teams by lowering the expansion fee. This is their end game - boost their franchise values up an order of magnitude.
I think 32 is basically the high end for the league long term. And it will take longer than you might expect to get there. From 24 to 28 might take 5-10 years. Going from 28 to 32 might take 10+ years.
As for pro/reg? Seems like a long shot. A group of lower tier teams are going to have to buy that right for a hefty, hefty fee, because it will instantly take down the value of MLS franchises 10% to 50%. Let's say in 15 years there is a thriving NASL with a decent TV deal, so the relegation threat only devalues MLS franchises 15%. There are 28 MLS teams, valued at an average of $750 million (compared to today's ~$100 million). That's $3.15 billion in value. That's how much the NASL would have to pay MLS to adopt pro-reg between the levels. And that's only if MLS owners decide they want the cash. More likely, NASL would have to pay a premium, so you're looking at a merger that costs NASL $5+ billion under this scenario.
A perennially successful team like the Cosmo's could see their franchise value double, as they expect promotion and to retain their MLS spot for a while. But smaller NASL fish are not going to want to contribute their $200+ million to the pot just to have the chance of getting promoted one day.