you don't replace players like Silva, Fernandinho, eventually Sane, Mendy, Gundogan with bunch of kids and continue to win multiple trophies. It's hard to replace such players with world class players, as you never know how they will adapt, let alone kids.
Kids knocking on the first team door will never be main source of new players at clubs like City are. One in 100 bluemoons will become regular, the rest will be sold for profit or be used as backup. They can be, of course, but we'd go Arsenal route, not domination one.