People have a curiosity to visit the worlds that they create. Few people have much curiosity to visit worlds that Thomas Hughes created, and in fact he does not do much creation. The act of writing realism is not one of creating worlds but merely describing them well. Yes, they have been, and will continue to be so for as long as they persevere with Potter. But thirty-five million book buyers seem to think they are enriching their lives with Potter.
Popularity is no sign of elevated literature, of course.
Trashy novels frequently are best-sellers. But they almost always pander to sexual curiosity or a fascination with violence that is not present in the Potter books. They are finding some reward. I think it is fantasy and fun. I can understand that viewpoint.