As far as the Narnia thing goes, as far as I know at any rate, there isn't any definitive proof that chronological order is what C.S. Lewis would have preferred. From what I've heard, that comes from one correspondence from one little girl who told him that she had decided to switch the books around and read them that way, and he told her he had never thought of that before and that it was a good idea. And apparently his estate picked up on that and decided, based on that one comment, to fuck everything around.
But, yeah, The Magician's Nephew is pure exposition, pure prequel, pure sequel. I mean, its opening line is something along the lines of, "This is the story of how Narnia came to be." What the hell is Narnia? Why should I care how it came to be? Well, if I've read five other books about the place, then that's a pretty compelling reason, and that's obviously the context under which Lewis wrote that opening and that he expected his readers to have when he wrote it.
As for The Lion, the Witch, and the Wardrobe, there's a reason it's consistently been the first movie adaptation of the series to be made. It's the most popular, it's the most well-known. It's probably the only one that's made such a strong impact on popular culture. It's the one that everyone has read. It's the one that I read and watched as a young child before I ever knew there were any other books. Just like the original Star Wars, it stands alone as a separate story while all the other sequels simply do not and cannot.