Children's Books as Movies: Where is the imagination going?

One of the big reasons why I want to work with older elementary students is because they read and interact with many different types of literature. I am especially excited about working with novels with my students. The ability to read a longer story and get to know the characters and the setting in such a deeper way makes the reading experience that much more exciting. I spend most of my free reading time these days reading novels suitable for this age group. Percy Jackson and the Olympians was the most recent of these books that I have completed. I honestly feel like I learned more about the Greek Gods and Goddesses through this book than I did during high school! The book was engaging and made my imagination run wild!
When I sit in my fourth grade placement class during reading time and watch students delves into The Diary of a Wimpy Kid, The Tale of Despereaux, Harry Potter and Tuck Everlasting, it makes me excited to see them enthralled by great literature that can span generations. I think that these books are great opportunities for parents and children to love literature together!
But here in lies the problem. More and more of these books are being turned into major motion picture events. I just saw the preview for The Diary of a Wimpy Kid: The Movie. One of the greatest things about reading is that you can tell the story in your imagination. Based on the authors descriptions, you can make the characters look like anyone you want, the locations are your own and you can create a movie in your mind that is all your own. Now, with more of these books actually being made into movies, children have a cast designated for them. They can see someone's vision of Hogwarts castle or Depereaux's dungeon and are shown a casting director's image of Winnie Foster and Greg Heffley.
Does this kill the imagination for these young readers? Are they still able to make their own movies or has that thrilled been taken away from them? I know that for myself, after watching a movie I see the movie characters as I read. Edward Cullen will forever take the form of Robert Pattinson no matter how hard I try to revert back to my image of the breath taking vampire from the Twilight Saga. Don't get me wrong, I have seen all of the Twilight movies and I am very excited to see the final installment of Harry Potter, but I had already completed all of the books before ever seeing these imposed images in my personal movies.
I worry that by taking so many books that are intended for young readers and making them into movies we are taking away a great experience from students. Will the Wimpy Kid series hold the same fascination for the next generation of young reader when they are able to get the movies from NetFlix? Will they have the same enjoyment I see my fourth grade boys get right now reading the series? I never think that the movie is ever as good as the book. The experience is just not the same. I worry that future generations will not experience laying in bed with a flashlight trying to read the last few chapters of a great book because they know they can just rent it this weekend.
I am all for a good movie, but I wish that some things were left to my own imagination.