Now, in theory, you learning from an author's works and competing with them in the same market could meaningfully deprive them of income, but it's a very difficult argument to prove.
On the other hand, with AI companies it's an easier argument to make. If Anthropic trained on all of your books (which is somewhat likely if you're a fairly popular author) and you saw a substantial loss of income after the release of one of their better models (presumably because people are just using the LLM to write their own stories rather than buy your stuff), then it's a little bit easier to connect the dots. A company used your works to build a machine that competes with you, which arguably violates the fair use principle.
Gets to the very principle of copyright, which is that you shouldn't have to compete against "yourself" because someone copied you.
This is one of those mental gymnastics exercises that makes copyright law so obtuse and effectively unenforceable.
As an alternative, imagine a scriptwriter buys a textbook on orbital mechanics, while writing Gravity (2013). A large number of people watch the finished film, and learn something about orbital mechanics, therefore not needing the textbook anymore, causing a loss of revenue for the textbook author. Should the author be entitled to a percentage of Gravity's profit?
We'd be better off abolishing everything related to copyright and IP law alltogether. These laws might've made sense back in the days of the printing press but they're just nonsensical nowadays.
So here's the thing, I don't think a textbook author going against a purveyor of online courseware has much of a chance, nor do I think it should have much of a chance, because it probably lacks meaningful proof that their works made a contribution to the creation of the courseware. Would I feel differently if the textbook author could prove in court that a substantial amount of their material contributed to the creation of the courseware, and when I say "prove" I mean they had receipts to prove it? I think that's where things get murky. If you can actually prove that your works made a meaningful contribution to the thing that you're competing against, then maybe you have a point. The tricky part is defining meaningful. An individual author doesn't make a meaningful contribution to the training of an LLM, but a large number of popular and/or prolific numbers can.
You bring up a good point, interpretation of fair use is difficult, but at the end of the day I really don't think we should abolish copyright and IP altogether. I think it's a good thing that creative professionals have some security in knowing that they have legal protections against having to "compete against themselves"
That's a point I normally use to argue against authors being entitled to royalties on LLM outputs. An individual author's marginal contribution to an LLM is essentially nil, and could be removed from the training set with no meaningful impact on the model. It's only the accumulation of a very large amount of works that turns into a capable LLM.