Most people I know saw Lord of the Rings, where as I know maybe three people who have any desire at all to see Dune. Lord of the Rings was at least an extension of a book everyone and their dog read in school, and came with a certain charm. It was also very much billed as a massive, three-year-in-row Christmas event, from the jump. Yet, I don't know a single person who has ever read Dune (though, again, I'm not saying it's not popular), and as good as the movie looks, it also looks to be completely void of charm, while not having either a "summer" or "holiday" event feel either. It's just this random date in October, on the heels of three $50M+ opening weekends in a row.
Either way, I don't think you can really lump fantasy and sci-fi together. Between LOTR and Game of Thrones, blockbuster fantasy has gained a certain broad appeal/acceptance. But hardcore, uber-serious sci-fi, as this appears to be, definitely has a ceiling in terms of audience. It would be one thing if this was a *fun* "space movie," but it's not. I really do think the "weight" of it is a big turn off for most people. My family, for instance (parents, sisters, and their families) has seen all three LOTR movies, all the Star Wars movies, and both Guardians if the a Galaxy movies (the GOTG movies being their unanimous favorites), but when I showed them the Dune trailer a couple months back it was basically crickets. They just have zero interest in it. It's a small sample, I know, but speaks to a larger issue, and I'm afraid this franchise just isn't going to have any kind of mass appeal.