Julian Nagelsmann was fired by Bayern Munich when he couldn't meet the same expectations that Thomas Tuchel experienced this season, so if he returns to Bayern Munich and can't meet expectations again, I think he will receive another dismissal. So instead of experiencing disappointment a second time due to returning to Bayern Munich, it would be better if he stayed to continue coaching the German national team where he was more appreciated and his performance was quite good there.
I don't think this is a matter of being suitable or not suitable, what makes you think young coaches are not suitable to coach the national team? I think it is each individual's choice and choosing to continue their career.
Coaching the national team also doesn't lower his standards as a great coach, he still has a lot of time and and I don't think he will continue to coach the German national team, one day he will definitely coach a big club again.
Julian Nagelsmann is one of the best coaches in new generation line-up with his stint with Bayern Munich was good going, but we have big mistake by them, and they fired him which create more problems for them and at the end they were having worst ever end of the season in decade there were few offers for him, but he rejected them and having his sight at the German national soccer team which is also going good, and he can improve their quality which were going down in last few years with his own strategy and players skills.
Having young or old national coach has never been matter because here at this point technique and strategy is more important and things needed to be at the top for any coach, and he is doing good job with this team surely this will give him good experience, and he will be more improved while for his future jobs at any level because having coaching job at the top level is surely good, and it's not going to bring standard down of any coach.