People can pay for creation, not rent (i.e. patronage), or one could argue that there's a reasonable tradeoff with like a 5-10 year copyright. I don't think a 2016 or even 2000 book cutoff would materially affect the LLM training discussion. And research runs on government grants, so everything derived from public money is paid for already and should be public domain, meaning there is no knowledge cutoff for science, which is probably the more important area to have cutting edge knowledge.