Tolkien's work isn't just the published books, it's the lifetime of thought and scholarship that he put into Middle-earth. So of course there's a lot more to it than there is to Harry Potter. The linguistic element alone is sustaining a lot of professional scholarship. I've been a Tolkien fan most of my life and a sometime Tolkien fanfic writer for about a year; I would continue to be a Tolkien fan even if I never read or wrote another fanfic in my life. As you say, the relationship with the canon is a totally different one. A lot of Tolkien fans fell in love with the world, as much as with the characters, and in my own case, more.