I'm not sure I'd describe it as a plateau. It might be, but I'm not convinced. Improvements are definitely not as immediately obvious now, but how much of that is due to it being more difficult to accurately gauge intelligence above a certain point? Or even that the marginal real life utility of intelligence _itself_ starts to plateau?

A (bad) analogy would be that I can pretty easily tell the difference between a cat and an ape, and the differences in capability are blatantly obvious - but the improvement when going from IQ 70 to Einstein are much harder to assess and arguably not that useful for most tasks.

I tend to find that when I switch to a new model, it doesn't seem any better, but then at some point after using it for a few weeks I'll try to use the older model again and be quite surprised at how much worse it is.