Well his last year or so under Löw was a pretty good indicator of how much he could help Löw's team. If they were playing a well drilled possession game I'd be all for Müller returning. But they aren't and probably won't in the future either as long as there isn't a new coach.
I don't know. In a well drilled possession team, other players would flourish just as much. After all, Müller produces possession losses in abundance and did so for Löw's national team at the end of his tenure. And that team definitely played possession oriented football. I'd argue Müller's best phase occurred when he was played on the right between 2010 and 2014 so his competition would probably be Gnabry who's one of Löw's best performers.
Thing is, I don't really know if you can drill a national team to the point that Müller's short comings doesn't affect it too much now that he has so much competition. His last games for Germany were incredibly frustrating to watch. So wasteful in possession and so little end product because he couldn't create anything on his own. As good as he plays for Bayern currently, I'm not too keen on seeing those performances for the national team again.