Small clubs worry about the development of youngsters. If Rio was at West Ham, for example, then West Ham would worry about his development so that they could develop him and be able to sell him.
Big clubs do not worry about the development of youngsters. Big clubs just provide opportunities for good, young players to play and to COMPETE with other big and great players. One cannot worry about Rio's "development." Slot cannot worry about Rio's "development." If he were to do that then he would not worry about winning the league. Ngumoha cannot be counted to be a substitute for the likes of Diaz and/or Gakpo. Simons, in contrast, could. Simons was top scorer in Holland (already, and at a time when he was 19 years old) and has had a great year at Leipzig, proving himself to be their top player at the age of 22.
Michael Owen was a great youngster that Liverpool once developed. But they didn't worry about his development. They let him play and he proved himself.
McManaman was also a great youngster that Liverpool once developed. But they didn't worry about his development. They let him play and he proved himself.
At big clubs you don't worry about the development of youngsters AT THE FIRST TEAM LEVEL. You sign them up, give them chances to play, loan them to prove themselves if they are not proving themselves at your club. And then bring them back or sell them. PSG have done that with Simons when he was 19. They've sold him to PSV with a buyback option. He was a top scorer in Holland.