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 has contributed greatly to Germany so far and won many awards so his decision to retire from the German national team can be understood considering that he is 34 years old and he wants more rest. There are still many young players who will replace him whose stamina is stronger and he might just Finishing his contract at Bayern Munich which will end next season, there are no rumors of clubs from other leagues wanting to recruit him, so I think after his contract expires, he won't move to any league. His loyalty is only to Bayern Munich and the German national team.
New talented players like you mentioned Jamal Musiala will carry on his legacy and inspire the next generation of German footballers.
Yes, Jamal Musiala will replace the role of a senior player, he has learned a lot in developing his performance. I saw his performance at the Euros this season and it was extraordinary. He is also an important player for Bayern Munich, with the skills he has, Germany is not worried even if they have to lose their senior players.