I read that Frank Herbert wrote Messiah to counter the impression that Paul was a hero. He wanted to show what happens when people fanatically follow someone. But there is already an example in Adolf Hitler. Messiah was not as popular as Dune perhaps because it's dark. The purpose of a movie is to make money, not to strictly follow a book. Most of the audience for Dune Part 3 will have never read Dune or Messiah. We will have to see how well Dune Part 3 performs against other 2026 movies. It will have a lot of competition, Avengers: Doomsday, Odyssey, and Spider-Man: Brand New Day to name a few.