But then you have to train them outside position when they have a high salary. U21 would still get players, although their potential will not be enough for NT.
if you stop training main skills in the first 4 years you will have uncomplete players for european competition and u21 team will decrease its strenght and would not be able to compete at maximum of its possibilities.
if you think anyway that's a good choice for getting sustainable salary players in order to create stronger senior national team players without going bankrupt I don't agree but i can accept it. you give up u21 team results for hypothetical better results in the senior team.
One more thing: I'm Italy u21 and NT teams coach assistant and I can ensure you that 99% of players who get senior team have been called by u21 team and trained at their best in first four seasons. other skills have been developed once they're 22yo. Similarly for Iceland and Nigeria, both teams I did look after in the past.