I think the writing part is the crux of the matter here - just going by how RJ described his creative process in various interviews, and by the gaps between his movies. At this point if he doesn’t do IX, I doubt he’d be writing it either. And if he does direct it, it will be his script.
Plus there’s the part where they have to scrap whatever pre-production they managed to have that was based on several aborted scripts. Alien 3, anyone?