In the early 2000s you always saw young players develop at a team. It wasn’t just youth academy, it was guys like Ronaldo for example who developed at man united. Barcelona was full of youth academy guys. Arsenal used to build up young players. Now teams never build guys up they just buy them.
That’s just wrong though
Arsenal is building up Saliba, Martinelli, Saka. Barcelona has Balde, Gavi, Yamal and others in the roster. Man Utd has Garnacho.
Plus you mention now they buy them, but Ronaldo was bought by Man Utd (like some I mentioned).