It gives me pause too, but this is the Harry Potter series. One of the only pop culture phenoms that actually managed to stick the landing and have a satisfying ending. (Hermoine belongs with Harry, but I digress.) Lots of other shows, think of GoT specifically, didn't have the full story with the proper ending. And word is that HBO wanted to keep going for more seasons despite the cost, but the Davids wanted to move on so they wrapped it up too quickly.
If any show can be sustained for that long, making enough money to pay for itself, it's the Harry Potter books. Even with all the stuff surrounding Rowling, the video game was a huge financial success and there are two very crowded theme parks based on it.