It’s not the casting, or even the likely changes to the plot. It’s the perennial problem with DUNE: it’s a trilogy that apparently can’t be made as a trilogy. You either end up compressing it down into a single flick, or you make a miniseries that doesn’t let you do the sweeping cinematic panoramas on a huge movie screen.
But they’re gonna keep trying, apparently. It’s like I AM LEGEND: some projects are just catnip to ambitious directors. And, heck, I’m probably gonna go see it anyway, so: good luck!