That 7am departure is the only practical flight for anyone who needs to be in PHX that day for business or whatever reason. This could mean a higher number of elites on that route, perhaps even people overnighting from other cities out East or from later arriving Europe flights. I concur that you probably wouldn't face much in the way of connecting passengers, though, so you're primarily looking at O/D traffic, and I've got no idea about the O/D traffic between DTW and PHX.