I think your definitions of mainstream casual whatever are too rigid. Fact is this mod has nearly 2 mio players. A mod which is buggy as hell, has incredible complex controls, which game play is unforgiving, hell even the installation process used to be a challenge on its own. Everything suggests this mod should indeed be a niche product with only a small hardcore fanbase, except that its not . 2 Mio players is hardly niche, nearly every gamer knows about dayz, there are youtubers who make a living streaming dayz, it even allowed a scam product (warz) to cash in on this popularity and its "developers" surly made a nice sum of it. The interesting thing about dayz is that it is a hardcore mod of a hardcore game which reached massive popularity, it became huge much to the surprise of everyone