My point is that it's the same with movies that are based on pre-existing books. There's always the risk that the movie will be a disappointment compared to the book; indeed, that arguably happens more often than not (with notable exceptions like Jaws, The Godfather, Blade Runner, and Who Framed Roger Rabbit?, which are generally considered as good as or better than their source novels). No matter which comes out first, there's no guarantee that you'll always find the movie as satisfying as the book. That's why it's good that there are two versions. It's just a matter of appreciating each one on its own merits.
(This is why I've never understood the mentality "Wow, I loved that book, I hope it gets made into a movie." Given how often movie adaptations are inferior to the books they're based on, or severely trimmed down or substantially rewritten, is that really something to wish for? If you love the book, it makes more sense just to appreciate the book for what it is than to yearn for it to be changed into something certain to be different and not necessarily any good.)
The makers of The Empire Strikes Back didn't mind that the novelization was released a month before the movie, even though it gave away the twist about Darth Vader. And I think David Prowse gave it away in an interview something like a year before the film came out.
I think it's a function of people reading less these days. Back when people read books all the time, and movies adapted from books or well-known plays were a big part of the industry, it was common for people to know the story of a movie in advance, so it wasn't seen as a problem if they did.