6 months ago:
Some things don't need to be written out. The story is very much "closed;" it doesn't have any loose ends to wrap up on. To write another book, I think, would drag it out when it really doesn't need to be. I have seen several things that I liked at first, so I went to see (or read or hear) the next parts, and it ruined it for me. Now, it would be hard to ruin Harry Potter (I'd say impossible), but I just firmly believe it does not need a "epilogue book."