There's that, but there is also year of fan interpretation which leaves things even more inflexible. I can't wait for the Amazon LOTR prequel series to drop and watch the fan reaction to that. Spoiler alert: it will not be pretty.The problem is, if they want to explore anything before the original book, they're probably going to be stuck having to adapt the KJA/BH books. They seem to be the ones in charge of the overall franchise and I'm not sure if they're the kind of guys who would let someone go off and do their own prequel that doesn't use their books.
Dune will be equally considered catastrophic.