I think you're right, it is partly a response to the Potter books. Having worked at bookstores from just before the first HP came out through #6, I can tell you that the amount & publication dates of fantasy for children has changed considerably. Barring some DWJones, and DDuane, with some of the Redwall books for good measure, most of the children's fantasy on the shelves was stuff that I (nearly 1/2 century, here) grew up with, and it's largely out of date & confusing for younger children. Edward Eager, who I love, but the kind of life his characters have, from around the WWII era is pretty well gone, just like the kids in the Narnia books. I think that tha's why the Narnia adaptations haven't done any better than they have, to be honest. The kids think of them as 'historical with fantasy' rather than simply fantasy.