You are often allowed to nake a digital copy of a physical work you bought. There are tons of used, physical works thay would be good for training LLM's. They'd also be good for training OCR which could do many things, including improve book scanning for training.
This could be reduced to a single act of book destruction per copyrighted work or made unnecessary if copyright law allowed us to share others' works digitally with their licensed customers. Ex: people who own a physical copy or a license to one. Obviously, the implementation could get complex but we wouldn't have to destroy books very often.