Shouldn't any player who is selectable for the U21 be playing for the U21 side? Even though they have played for the full side? Other countries do that.
If you're a fringe player then probably but if you're a starter or make regular appearances for the senior squad then there's no point them joining the U21s. Looking at some of the senior squads, there are quite a number of players who are eligible for the U21s but apart from Bellingham and Musiala, I don't think any of those players have any business playing for the U21s.
England: James, Rice, Foden, Mount and Bellingham
Portugal: Pedro Neto and Joao Felix
France: Mbappe
Germany: Havertz and Musiala