It’s Rowling’s business why she writes those books, I don’t care actually. But I do not think that without IP privileges she would be so much poorer that she stopped writing. Sure, in future people will buy physical books less and less, that’s inevitable. And probably there will be less people who write books. Just like there are not so many people anymore profiting from horse-drawn vehicles since the invention of the cars.
Just because the unfair (to say the least) privilege from the state created for people incentive (that’s questionable) to write books doesn’t mean that is right or just according to libertarian principles.
The reason I say that’s questionable is because people still write today many fiction and non fiction books or stories for free and they still are able to profit from it via donations etc. No need for IP privileges.