From my humble point of view these things can only happen in small countries and in this historical moment.
Before Utopia's dual citizenship, small nations had a very small drafting area. And it often happened that we had to train players with ridiculous potential to be able to bring them into national teams.
With the advent of dual citizenship, many more players with decent potential have arrived in small countries, moreover almost always in the hands of managers with above-average experience.
So it would be a question of changing a common sense rule for a small number of seasons. In my opinion it's not worth it.
As noted by others, it is not said that the "best" players of the U21s change the results of the senior national team in an overbearing way.
Anche tu a caccia di "Riconoscimenti"?! Vieni in federazione: Achievements hunters (fedid=19269) (solo per supporter, se non lo sei e hai dubbi chiedi pure, spero di poter esserti di aiuto)