Once Harry Potter is published, it's out there and it's done. The series lies in those printed novels. That doesn't change. If Rowling wants to let people add more, she can, but it is certainly not a requirement.
Similarly, I absolutely have the right to suspend a software project of mine and leave it hanging, if that's what I want. Perhaps I don't want something associated with my name being sullied by other people's work. Maybe I want it to be associated with me and only me.
The fan was terrific, but he tried to make money. Rowling told him she was against that and he did it anyway. So it wasn't like she lied to him.
By the way: I downvote people as soon as they ask about why they're being downvoted, unless they're giving a good rationale for why they shouldn't be downvoted. Maybe that's why the negativism?