First, Bronco can still take over the defense and leave Howell with the DC title. Bronco couldn't take over the offense and do the same with Doman.
As far as staying on as QB coach, I seriously doubt Anae wanted Doman there. It would just be a daily awkward situation. You have to remember that Doman probably did his fair share of things to help usher Robert out the door. That creates some bad blood. It's best for everybody to part ways and keep things in order.