Clubs that develop their own players, trust them & risk relying on them from time to time, because it is important.
If you look at what you have posted, then it means no young player will ever play for City. We can always find a player who, if we don't sign them, we could be said to be 'missing out' on strengthening, as opposed to promoting a kid. Always. Even when Messi was a kid, he wasn't the absolute best player Barca could have had. He was used because he was good enough to play a part not because he was perfect. Same with Giggs at Utd. He wasn't the best winger in the league at 19 there were better, older players. They could afford to sign better. None of those kids were the best. They chose to trust them & develop them. And won the title.