Book to movie adaptions

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when comparing books to movies, I prefer books. You can use the words to create your own imagery and the details provided by a single author allow your own interpretation. Once a book becomes a movie- it is usually redrafted and watered down to have a mass appeal. Also, it is very challenging to ensure the original fan base isn’t alienated while trying to recruit a wider market.
Many popular books become even more popular once transferred to a movie series though. The Hunger Games, Harry Potter and Lord of The Rings are a few examples. Sometimes the movie becomes so popular it is not common knowledge that its origin was a book. Almost all Stephen King movies come from his books. Stand By Me” was actually from his book “The Body”.”Fight Club” starting Brad Pitt was actually a book as well.
I have always been curious how a book gets picked to be a movie. Comic books seem to be very hit-or-miss in terms of success as a movie from a popular franchise. Action books like Hunger Games & LOTR have done well, but it seems like romantic comedies and/or chick lit books don’t do as well.
It seems like an overwhelming amount of movies have origin as books but the tie in seems to die down once the movie is released. I find that once a book-to-movie adaption is released, the book is re-vamped with a different edition or more similar to the movie than the book. With the rise in fan-lit and audiobooks will there be a need for original screenplays?