I agree with
@Peat here. A reader is allowed to be disappointed because something that resonated wildly with her in the movie isn't there at all in the books. Yes, Eowyn is a fantastic character. Yes, I always loved her, from the first time I read the books at thirteen right up through my absolute fave Miranda Otto doing so wonderfully in the role. But for Samantha Shannon,
Arwen resonated - a different sort of female character, doing a different sort of thing. Maybe, for her, what was amazing wasn't the "woman standing up and doing the male job" but the idea of a woman being in command of amazing forces of magic. That's FINE. That's her own thing.
She wasn't "looking for female role models in Lord of the Rings". She was having an emotional reaction to experiencing the story. She's allowed to do that. It's the entire point of stories.
Also, none of that changes the mildly outrageous fact that there are a couple dozen named male characters doing different things in different ways in the Lord of the Rings, but we're saying, "Why is she complaining about that woman not doing stuff? There's A WHOLE OTHER WOMAN DOING STUFF." Seriously? Don't complain, there's another one? Gosh.
Sorry, I'm really not wanting to jump down anyone's throat here, but this entire thing is exactly why Shannon went, "I want to write fantasy fiction with lots of women." And more power to her. I love Eowyn, but I still want to write (and read!) fantasy fiction with lots of women.