I thought the route is MLS Next (youth) > MLS Next Pro (2nd teams) > MLS. If you look like you could make it, you get a run with MLS Next Pro and if you exceed some bar, then you can get signed to that level. If not, then college ball or bust ...
You also have USL Premier, L1 & L2, so its starting to expand.
The MLS closed league is a fundamental problem with developing a healthy soccer culture & infrastructure, although changing that will not drive youth development into the pros, it's more likely to have the opposite effect as owners/coaches are not going to chance a youth prospect when the downside is relegation. Bizarrely, the MLS closed system should incentivize, to some extent, teams to promote/develop youth players as there is no fear of relegation.
The number of academy kids that make it in the pro leagues in Europe is tiny. Making an academy at a young age gives you a minimal advantage. Some large clubs use academies to generate money to make them self funding and obv. hope to get some players.
Alternatively, you can build a team if you "require" yourself to restrict your choice ... Athletic Club Bilbao being a stellar example.