Can I get an AI to eventually write another book in Terry Pratchett's style? Would his estate be entitled to some form of compensation?
No, that's fair use under parody. Weird Al isn't compensating other artists for parody, so the creators of OpenAI shouldn't either, just because their bot can make something that sounds like Pratchett or anyone else. I wrote a short story a while back that my friend said sounded like if Douglas Adams wrote dystopian fiction. Do I owe the Adams' estate if I were to publish it? The same goes for photography and art. If I take a picture of a pastel wall that happens to have an awkward person standing in front of it, do I owe Wes Anderson compensation? This is how we have to look at it. What's good for the goose must be good for the gander. I can't justify punishing AI research and learning for doing the same things that humans already do.