You raise some interesting points about storytelling in different mediums, and I'll do my best to address those, but I think a point that's often overlooked in book v. film discussions is that too many people spend far too much time worrying about "accuracy" and not nearly enough time worrying about meaning and aesthetic value.
However, to your point, if a film or television show makes the claim that their material is "based on the such-and-such trilogy," then they should operate with at least a modicum of loyalty to their source material. To that end though, the demands and realities of screenwriting for film and writing for novels have to also be taken into account. When a single writer is sitting at her desk creating a story intended to be printed, she isn't worried about which actors will play which characters or how difficult it may be to render each setting onto the screen. She's just writing a story. Later on, when her stories become world-wide bestselling children's novels, people in the film industry then have to make those considerations. So I also think we should give a bit of autonomy to the screenwriters and show runners whose jobs are to take this individually conjured complicated masterpiece and turn it into something which--hopefully--successfully is perceived by viewers as "authentic," "loyal," and meaningful.
I probably make a similar point about most subjects, whether it's a severely important political matter or a more banal matter like book v. film, but reality is extremely complicated. Humans are complicated, and every issue we discuss is also complicated. I tend to be skeptical of oversimplifications and generalizations. Books aren't always better than their movies. What matters is how the material (book or film) is received by its audience.
We also have to consider the infinitely nuanced and unrealistic expectations that readers of stories carry with them into viewing a film adaptation of a text. (i.e "That character was killed by being stabbed in the chest with an enchanted Dragonsword in the book, but in the movie some troll just cut his throat with a mythril dagger!") Who gives a shit?! I certainly don't. The questions I try to ask myself when watching a film or TV show are: Is it entertaining? Is there character development? Can I learn something from empathizing with any of the characters' experiences? Is it trying to make a statement metaphorically, and if so is it salient? Last on my mind is "did they follow every pedantic plot point exactly?"
The amazing and seemingly magical thing about books is that every time another reader picks up a book and starts reading, and entirely different world emerges in her head, unique from any other reader's experience. Do we honestly expect screenwriters and filmmakers to accommodate every reader's experience?
So yes, filmmakers and screenwriters, in the event that they expressly say they are adapting a text for the screen, have some obligation to at least attempt to stay loyal to the source text. However, viewers should keep in mind that the screen is an extremely different medium than the page. What works well and seems beautiful in prose may come off flat and boring when viewed on a television or in a theater. Try to keep an open mind and low expectations with film (and probably books as well) and you'll probably have a much better time.