> Once you've trained on the internet and most published books (and more...) what else is there to do? You can't scale up massively anymore.

Dataset size is not relevant to predicting the loss threshold of LLMs. You can keep pushing loss down by using the same sized dataset, but increasingly larger models.

Or augment the dataset using RLHF, which provides an "infinite" dataset to train LLMs on. Limited by the capabilities of the scoring model which, of course, you can scale the scoring model infinitely so again the limit isn't dataset size but training compute.