I have no comment on the fact of the characters skin tones. I have not read the books and attributed no images to characters so it doesn't make me upset. I am commenting on Sanderson's mention that one MUST change source material to make a proper adaptation. I disagree. (he said it was good to disagree :P) I will trust Sanderson on the skin tone thing until I have a reason not to. (I mean he did help write the story after-all LOL)
I don't think that one necessarily HAS to deviate from source material to make a story compelling. Whereas I completely agree that a filmmaker must put his/her own stamp on a film, I have seen several examples how deviating for a story can HARM a film. Look to Game of Thrones season 8. Season 1 of Game of Thrones was a dang near perfect adaptation, only cutting extremely minor characters and subplots, but it still had D&D's heavy-handed style. Most fans (that I have talked to) agree that its after the show started to deviate from the books when the show started to get more contradictory, and less exciting. (and therefore harder to escape into and suspend disbelief!)
I agree that the 3rd Harry Potter film is the best one, not because it deviates, but simply because it told the best; it has the best filmmaking; it does the best at persuading the viewer to fall into the whimsy of the Magical World.
Also, Cuaron put so much of his style into places where it fit perfectly(especially using the dementors to showcase his signature dark-heavy contrast.) So, its not that the first two films were badly adapted so-to-speak, (I know a lot of people who cherish them) but several audiences admittedly found them to be stale due to the stagnant direction. But that is comparing Chris Columbus to Alfonso Cuaron, hardly a competition. I guarantee that if the third film had followed the book more closely while retaining its sharp aesthetic and pacing (maybe with a bigger budget and a studio willing to put out a longer film or miniseries even) it would be better.
I mean look at the 7th Harry Potter book adaptation, which many people love. It was almost a page to film adaptation and it worked because of its dark and menacing tone. Many people say the worst HP film is the 6th one (because it left out too much and much of the movie seemed pointless; hilarious, but pointless).
Taking a look at King Arthur's often troubled films, one can see that most of them become forgettable after a couple of years. The most stand-out of these is arguably John Boorman's Excalibur (or Disney's bastardization xD). Boorman's Excalibur, while taking liberties, is definitely the most close to Le Morte de Artur (well at least a lot closer than Fuqua's, which went for Historical accuracy over adapting a fictional story, and definitely Ritchie's, which was pitched as a direct adaptation with one movie per Knight of the Round Table but the producers got cold feet so it became a mediocre film.) I would argue that most people who try to adapt King Arthur would do a lot better if they just followed the already compelling and thrilling storyline present by Mallory!
Yes it is true that often when Hollywood turns a book into a film with the same author as screenwriter, major changes were made. (You mentioned A Princess Bride; Last Unicorn, Willy Wonka and the Chocolate Factory) I think a lot of this has to due with the production of the film, not the artistic integrity of the people involved. Now I might be wrong, but if budget were not an issue, I bet most adaptations from this time probably would have adapted some of the more pricey, effect heavy sequences from their own books.
I don't see why people cannot make adaptations of Books into films and try to replicate the book while giving its their own visual style, and still not change a thing. A great example is the Fellowship of the Ring. Yes there were HUGE cuts made to streamline the narrative into a serviceable film length (3 hours :P) but if your look at both of its adaptations, the directors (Jackson and Bakshi) gave so much of their own personal styles into the films without really changing much. (generally people say that Fellowship is the most direct of Jackson's adaptations. Especially considering the Hobbit films)
A lot of people who have read the in's-and-out's of the a might welcome a few deviations to keep it fresh and exciting. Others, who have never read the books might not even notice and therefore do not care. I am in a third category when it comes to Wheel of Time. I have always wanted to read the books but haven't had the time. I also LOVE films and filmmaking in general. I want this to be adapted correctly so that it uplifts the source material instead of changes it. I don't see why my two loves (filmmaking and fantasy) must fight every time they meet!
I think that several critics have the notion that to showcase a director's style or to make a good film adapted from a book that it must be different from the book and I disagree. I think it all comes down to the way the story is told.