I make this point every cycle of international tournaments, if we play a full team of the best U21s we would qualify more regularly for the finals. We would then have a cohort of players who were used to going to summer finals if/when they emerge as full internationalists.
I first noticed this when Stephen Pearson was withdrawn from an effective play off match for the U21s, which they lost, to sit on the bench for the full team, although my argument fell down a little as he got some game time. But there may have been more benefit overall if he had helped the U21 team qualify.