I can't help but wonder something.
How many have actually read the book as opposed to basing their knowledge on the movie and / or mini series? And of course when I say book I appreciate the plethora of sequels, but for the purposes of this we're talking Dune.
I agree that one movie wouldn't be able to impart all of the book so they have a choice; cut loads out and make a movie with just the main elements in it, or span it out over a trilogy. Personally I'd go the former, I'm sick of dire films made from excellent starting concepts (books, comics etc) and I'd rather appreciate a good film with bits hacked out from the book. It'd be more likely made into a trilogy anyway and there's loads of material to continue forward, and some excellent films don't follow the book accurately but are still watchable.
Finally although it was along time ago when I read it , and I was a lot younger, I know I had to frequently refer back and reread bits when I'd lost the plot. Doesn't quite work out the same if you have to do that with a film.
Anyway we all know why Dune failed, it lacked Ewoks or a Jar-Jar character for fans to attach too....