Robben is also old. I know he is still a great and impactful player. But at his age, if you can get a successor, no matter where he plays, you get him. Though, there would still be a place for Robben. He could obviously play for Müller. It could also very well be like this:
-----Lewa-----Müller-----
James------------Robben
-----Thiago----Vidal------
Alaba-Humm-Boa-Kimmich
This would work quite well, too. Alaba would provide the needed width so James could roam more in the middle and Robben could play on the line with Kimmich playing more in the middle (like Lahm did for Pep).
James provides us with loads of options. I just think the 4321 is most likely, as it is Carlo's favorite formation.
By the way, Robben was benched in favour of Müller before. He was just lucky Kroos got injured. Otherwise, he might have spent the tripple on the bench.