North Texas SC was never intended to be a career in and of itself. Players either develop or leave. If you develop enough, you’ll earn a shot with the first team. If you don’t, you leave anyway. Probably the closest analogy is large companies like Apple or GE or McKinsey sponsoring students to go get their MBAs, except if you don’t do well there’s no guaranteed job with your sponsor afterward. U23 teams are soccer grad school, and the end of the season is graduation day.
My assumption is that the upcoming MLS U23 league (in which FCD’s second team will compete) will have similar roster rules as the U23 Premier League in England.