First off, there’s no question that film one will be a massive moneymaker for all involved. If it’s decent to good, then two more gravy trains will follow. It’s not like JK Rowling needs the money. I wonder what the process of writing has been like for her to reportedly polish off a movie script in twelve days compared to however long she works on her adult novels.
Maybe she’s just a greatly-gifted fantasy writer. And that’s fine.
If I were in her shoes, I would take a nice long retreat and explore some alternate fantasy realm about which to write. The genre is great.