Here's the thing. If you write something that becomes as huge as Harry Potter, when you're done, you have three options. The first is to keep writing, but write other stuff. The second is to keep writing, but do HP stuff. The third option is to not write anymore.
If you write other stuff, people will ALWAYS compare it to HP. Some might even ask why she'd write other stuff when there's still a lot of HP to explore. If she writes HP stuff, she's a sellout that just want to milk the franchise. And if she stops writing... Well, she's not really an active writer anymore. It's easy to say you should quit when you're at your peak, it's a whole different story actually doing it.