What pressure is he under? The vast majority of people who watch the films haven't read the books and few if any are going to be 'I wonder what happens next?' given the principal storyline wraps itself up nicely. The good guys beat the bad guys.
I'm well aware of Dune Messiah...I've read all the books (albeit in my youth) and honestly, the fact that Herbert was pressured into Messiah showed given both its shortest and overall paucity (The negative response in many ways then led him to course correct with Children of Dune which was a lot better). I watched a
YouTube recap of it the other day to refamiliarise myself with it and it's not got 'cinematic experience' written all over it, versus court drama for the most part. Although I really enjoyed the first film and am looking forward to the sequel, it would have made a lot more sense realistically to have made Dune as a big-budget HBO series in the spirit of Game of Thrones. As great as the first film was, compared to the books it was a bit too broad brush at times and definitely would have benefitted from more build-up, over say an 8-10 episode season of Television. Honestly, It barely feels like The Atriedes have taken over Arrakis before they get attacked out of the blue by the Emperor and the Harkonnens, plus also an HBO show wouldn't be hampered by the need to qualify as PG13, whereas the book series most certainly is not.
DV is a fantastic director and honestly so far hasn't put a foot wrong, but I'd be more inclined to move on to other projects at this juncture if I were him versus trying to push a project that has every likelihood of flatlining the franchise entirely because Messiah is the runt of the litter at the end of the day. Alternatively, if there was/is enough additional footage to turn the 2 films into an HBO adaptation with more added scenes, release that and then make Messiah as a series, because it's a lot of plotting and scheming for the most part, and TV excels at that sort of thing over film.