I heard an interesting story that once MLS and USL are done expanding, they plan on converting local youth leagues around the area where their teams play to SYL leagues. Apparently the SYL sees itself as the most elite youth league outside of USSDA..
http://www.sylsoccer.com/
https://www.soccernation.com/presidio-soccer-league-meets-mls-stadium-project-investors/
http://www.sylsoccer.com/
https://www.soccernation.com/presidio-soccer-league-meets-mls-stadium-project-investors/