I totally agree with you. Thomas Muller has done so much for German football and now it is time to give younger players chance. He has had amazing career winning many awards including the Golden Boot. He is legend in German football. He might keep playing club football but he will probably stay with Bayern Munich until his contract ends. He might consider playing outside Europe but it is unlikely he will join another European club. Muller time playing club football might be coming to an end but his legacy will live on. New talented players like you mentioned Jamal Musiala will carry on his legacy and inspire the next generation of German footballers.
Germany will missed out on one of the greatest player of all time, Thomas Muller. He's basically an important instrument for his club and country, although he's aged and no longer active in essential sectors, more comparable to pressing force. Thomas Muller have made public announcement, he has finally hang his boots when it comes to international games. He retire from international games and having full concentration on the big games at club level.