I blame Tapatalk's bad date placements ;( Now I have become Necromancer! I got carried away watching people discussing this stuff elsewhere I felt I had to let out my inner mathematician!

I suppose at least it stimulated discussion during E40K's anniversary year!
I would actually say that Dune is not 100% hard SciFi, Jimmy, at least in the sense I meant: I suppose there are more learnèd opinions out there, but I would say 'hard SciFi' attempts to be as realistic as possible given what we know of the universe (in addition to other SciFi things). Of course, there is a spectrum in that definition, and Dune is a lot 'harder' than your average fantasy book! And quality doesn't have to depend on 'hardness' either, of course.
Blip, I think the pervasive nature of sci-fantasy is something deeper than that (even classics like Buck Rogers are pretty Space Opera-ish, despite the lack of laser swords): the future always seems to be a popular direction for fantastic imaginings, and modern times have allowed fantasy writers to explore different genres to those traditional ones. The roots of realism in SciFi I suppose come from the selective realism in Victorian Gothic horror (and Victorian literature often had some elements of reality central to the story—some kind of hidden gnosis or innovation. Very zeitgeist-y

).
But I think 'true' SciFi (no matter how 'hard' in realism terms) has always been the thinking human's purview: the average person generally doesn't like such cerebral reading these days, it seems. And if you're going to make a far-future universe, you already have to make a bucket-load of assumptions if you imagine such ridiculous things as 'stable interplanetary governments'! David Brin is an interesting case, being an actual employed futurist and physicist in addition to being a harder SciFi writer—his Uplift books have plenty of physics but also plenty of phantasy in FTL travel, and he talks about it.
Continuing the long discussion of literary theory already contained herein

I'm glad I'm not the only one who is concerned with the theory that makes the 40K setting work! The moment you have a goodie that doesn't commit atrocities or die horribly (like Tau, resurgent Robute Guilliman...

), they don't fit very well into the dystopia. In a way, that's 40K's foundational principle which, as has been discussed many times, I'm sure, makes for a good game about people killing each other!
(swirls around the magic of Undeath once again...)