It is kinda a meme at this point, that there is no more "publicly available"... cough... training data. And while there have been massive breakthroughs in architecture, a lot of the progress of the last couple years has been ever more training for ever larger models.
So, at this point we either need (a) some previously "hidden" super-massive source of training data, or (b) another architectural breakthrough. Without either, this is a game of optimization, and the scaling curves are going to plateau really fast.