They'd be better off doing 40k IMO.
I think this would almost be a harder setting to put a game in, but if done well it would really be amazing. Given the xenophobia common among all the 40k races though it'd be hard to make peopple happy with it I think.
I'm not that familiar with the 40k universe, honestly. My comments stem more from the fact that it is far more distinct from the other games out there. You can call Warhammer dark fantasy, low fantasy, or whatever, but it is still fantasy. I LOVE fantasy, but that market is choked with titles. I think warhammer is different enough to put off many people that like fantasy, but not different enough to pull people that don't.
Moreover, they'll have to dramatically change some things, i think. The most dangerous opponents in the Warhammer Fantasy universe aren't monsters, much of the time, they are other people. This isn't a land replete with dragons and such that people are going to expect.