You actually can see this kind of herd mentality at play in other areas of life too. For example, some people buy only white and silver cars because it enhances the resale value (those being very popular colors that other people will find attractive). Hence, don't stray outside the circle designated as "aesthetically pleasing." What about house decorating? It's the same there too with neutral colors for walls and wooden floors because they are "in" right now. So it's natural that it spills over into other creative areas like book to film adaptations.
Just to be clear, I discovered long ago that straying from a book's plot really doesn't bother me. When Lost World had a T-Rex stomping all over a modern day city I didn't flinch. I thought it was cool and took it for what it was worth: entertainment. When the Hobbit took liberties with the script I didn't care at all. Hey, it was a story with this amazing dragon in it and some pretty impressive looking battles. Oh and it all blends itself so well with the Lord of the Rings trilogy now (with all that backstory). Great stuff. And now we come to "The Shannara Chronicles," which has Terry Brooks's blessing but my friend's Jake, Justin, and Sasha just railed about it this weekend.
"I'm horrified with what's going on with the Shannara chronicles," Jake said. I asked him "why" and then showed him my review that I wrote last week. His immediate comeback was, "You just like it because it features half-naked gorgeous guys in every scene." I was like...so? It's still a good story. There's this scary demon and the characters are visceral and real to me. The effects look dang cool...what's there not to like?
He started listing off his complaints: they screwed up Eretria, the trolls don't look like that in the book, and what's with all the decaying remnants of old Earth? None of that stuff was in those early books. It all came later. That's when you found out how the world collapsed. Also, he says that the elves are not descended from people. They are descended from fairy. In short, they've screwed up everything.
I just shook my head. I think he's wrong (of course) but I'm someone that can easily argue for why stories need to be changed and freshened up from their stale source material baked in an oven sometime in the eighties. Audiences today want young adult stories, and the Shannara chronicles is exactly that making its female protagonists front and center.
Do you think the outrage goes back to kindergarten when most of us were taught to color within the lines?