Mostly a good indicator I think
The German and Spanish teams of the last 10 years you can see their youth team progression
But a lot of it is based on who has developed physically too though, why African teams do really well too, and some countries aren't sending best players, the England team for example has I think 5 players who would be starting but already moved up to u19 or u21 level, I imagine a lot are the same
US looked dangerous though, some players from that team will definitely push on and be national team players