I accept that a closed league has attracted investment. But that's only in the short term because its structure promises a return on investment in the long term. This return on investment is the opposite of investment, it's taking money out of the US game.

Also, teams aren't going to automatically collapse just because they drop a tier. Some will but the majority will be smart enough to prepare for that eventuality. Once again, the converse of a team dropping to a lower division is that a team from a lower division will be going up enjoying the extra attention. Pro/rel provides a pragmatic way a wide variety of metropolitan areas will be exposed to top flight soccer. This is purely speculative but I suspect that teams facing a possible promotion will generate a lot of publicity. "Hey let's go to a Louisville City game, they play in a substandard division you've never heard of" vs "Hey let's go to a Louisville City game, if they win they are going up to MLS", which one is more enticing?

The other problem is that a closed league is predicated on limiting the number of teams and not giving a team to every market. This is for two reasons: it limits the number of slices the soccer pie is divided into making each slice bigger (i.e. more valuable) and it keeps alive the threat of relocation with which teams can extract public money out of local government. Having fewer teams and not covering the United States territory as densely means the youth system suffers. If you look at Spanish and German national teams essentially all of them started playing at youth teams of smaller clubs that populate the country's lower divisions. These teams would not exist in America, the pyramid structure is the only reason teams like Rot-Weiss Essen to exist and can train Mesut Ozil.

By upward mobility I meant in terms of division. An investor is going to be a lot less interested in investing in Louisville City FC if there's a glass ceiling on his ambition.