I think the relative failure of the German U21 also stems from filling the ranks of the main team. With these players being the first to have finished the revamped academy system in full, the quality raises and players jump over the U21. This problem will reduce with the first team having enough quality that these younger players can't usurp them before completing their time in the U21. Schürrle and Draxler for example are still eligible for the U21, as are Toni Kroos and Mario Götze (in theory at least, since both are inujured). Lewis Holtby is a good example of what quality his experience brings to the team.