The reason Soccer leagues (in Europe) have a reserve league is because they have nothing like the college system in the US. The teams like Man U, Chelsea, Barcelona, Real Madrid sign players as young as 8 years of age through their academies. The best then sign pro contracts at age 16, and unless exceptional play reserve soccer until they either break into first team squad or drop into lower league football.
It's exact same in Football. The best at high school go to college. The best college break to NFL or drop into Arena, Canada etc
I don't see the point trying to keep these guys for the odd player coming good. The majority would have showed it already
If they go to Canada or somewhere eg Cameron wake and earn a shot then great. Again in your original post you compared to soccer, it's like Ricky Lambert. Left Liverpool, played lower league and earned a shot.
It's not worth 32 teams of full squads tho. The college game is the development.
You are taking the reference too literally. There are american sports with college systems who use teams like this as well