Tomoyuki I'm not here to argue on that point, but let me point out three things:
HP is a book that has no designated market. It starts as the kids book suddenly turns into young adult novel only to end up as one dedicated for mature readers... But all of the books are still written in a way you would write a kid's book.
To make it even a better example, I hope you watched "How to train your dragon" series. The novel that the movies are based on, are simple kids book. But the directors turned it into a series that was perfect for you if you were a kid with the first part, teen during second part, and young adult during 3rd part airing. The problems that the each part of the movies tackled along with the way they were shown, escalated and resolved all lead to a satisfactory conclusion, with every element aimed to fit perfectly into the desired market audience.
On the other hand, you have Harry Potter, that inherently suffers from how it started with, because it makes it impossible for itself to grow out of it. Why?
- Harry Potter is a novel based on ordinary wish fulfilment premise. Not the one when you construct the entire book when basing on eastern wish fulfilment cliche, that is slapping you in the face with harem, money and all. Compared to eastern writing, this was the very first novel in western writing that dared to use those cheap tricks to gain the readers' attention.
In very first few chapters, MC that is shown as bullied by his own family, orphans, weak and all (made to make you sympathise with him) suddenly turns out to be someone with great potential, super amount of money and basically a descendant of the heroes.
The second Hagrid comes into the picture, all of the things that one would want to happen to himself, appears to be true. But while you could say - Hey, isn't that exactly the same in the easter cliche wish fulfilment novels?
- No. HP is inherently a western novel. A novel compete with series like LotR, Narnia, or literally any of the Sanderson's novels. It's a western novel that took something that we could call an eastern concept and shyed away from it when suddenly Rowling turned it into something more mature.
It's like if you was writing a harem novel where MC is the strongest since the beginning, only to suddenly turn it on its end and say that he was only dreaming about it all and was bullied along the way while living in the basement and having those dreams due to the drugs he consumed to the enjoyment of his bullying family.
3 - Harry Potter is a novel that was promoted extensively due to the concepts it introduced to the Western world. As this topic is quite deep and upseting for some, I will only direct you to learn something about it on your own, as I don't want to bring the hot potato of what it was aimed to achieve into the picture. Just the simplest thing, of the way it explained the magic turned the whole world on its head. Yet rather than giving you the answers, I want you to ask a few question.
Do you know whats the difference between magic in novels like Narnia or LotR? This intuitively a simple change, has an insanely deep implications that actually had a great influence over a great amount of young kids who read it, which showed in the way they had fun when the novel was first published and promoted. I encourage you to read more about it, as this topic is simply too big for me to bring it up in just a forum post.
*Yeah, I know I have shtload of grammar mistakes here, but I can't be bothered to fix them. Sorry for that ;P