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.
Tuchel was sacked by Bayern after a disastrous season in the Bundesliga, but with Negelsmann it was different, his results with Bayern were not disastrous. And Bayern invited Negelsman to return to the post of head coach, but he refused because at that moment he was the head coach of Germany, in fact, he now remains the coach of Germany, but it is not known what will happen after the loss of the Spanish national team, perhaps he will also be fired because a loss at the quarter-final stage at the home Euro may be perceived too negatively.