Welcome to the forum, andy!
Just look at the poll above and you will see that a vast majority likes the HP books - and they're not all teenagers. These books can even charm adults, "throw them in a different world", as you say. I get the impression that most of the critics that find fault with HP are LOTR fans (or some other fantasy books).
And as for the 7th volume being the last - who knows? Event Conan Doyle had to write another Sherlock Holmes story although he had even gone so far as to kill him!

