As much as I wish it were, I don't think this is clear at all... it's only been a couple months since Opus 4.5, after all, which many developers state was a major change compared to previous models.
The models are definitely continuing to improve; it's more of a question of whether we're reaching diminishing returns. It might make sense to spend $X billion to train a new model that's 100% better, but it makes much less sense to spend $X0 billion to train a new model that's 10% better. (Numbers all made up, obviously.)