The new AG will not be recused so he, not Rosenstein, will be the one to fire Mueller. (Rosenstein could do it now, but will not.). The selection of Rosenstein is another reason Sessions must go.
I would rather see Sessions dismiss Rosenstein and appoint a real junk yard dog from within the DOJ as interim Assistant AG who is over Mueller who will narrow down the mandate and put a deadline of October 1st on the investigation.