I think we have to be clear about what we are looking for from Miller. He seems pretty effective at holding up play and distributing ball on the deck. He is not terribly effective at directing headers to other strikers though, although he can win ball in the air. He is not a prolific goalscorer and that is the bit that we have to either come to terms with and find a place in the team for it or get rid.
Having watched the way McGhee seems to want us to play (and I am rubbish at this stuff as I'm usually too pissed at games to focus on formations etc) we are lookign for the goals and direct attacking play to come from the angles through the likes of Paton, Aluko, Maguire and Mackie. Similar to what Hibs do with Riordan and Stokes. Very few teams have an out-and-out penalty box striker these days, the forward roles have a different requirement. When Miller is not in the position of winning through balls and holding the ball up then I think we struggle to break down defences that are content to play soak and counter football. For this reason I would keep him but put the expectation onto others to deliver goals.