I suspect the possibility of an 'M' range of Dunharrow figures becoming available in due course has led to potential voters redeploying their support elsewhere. Why choose a more expensive Dead Man when more economically-priced figures might become available later?
(Note to Michael et al: Please don't interpret the apparent lack of support for the Dead Men as disinterest!)
Does anybody around here seriously believe that Mithril might consider or even release another M-series? I think the M-series are part of "Mithril history" by now. The grey primer might also be soon forgotten, I'm afraid.