I think GPT5 will tell if OpenAI hit a plateau.
Sam Altman has been quoted as claiming "GPT-3 had the intelligence of a toddler, GPT-4 was more similar to a smart high-schooler, and that the next generation will look to have PhD-level intelligence (in certain tasks)"
Notice the high degree of upselling based on vague claims of performance, and the fact that the jump from highschooler to PhD can very well be far less impressive than the jump from toddler to high schooler. In addition, notice the use of weasel words to frame expectations regarding "the next generation" to limit these gains to corner cases.
There's some degree of salesmanship in the way these models are presented, but even between the hyperboles you don't see claims of transformative changes.