That is, however, the argument at its core. We don't know where RSI would lead. It could produce something roughly human-intelligent. It could produce something slightly smarter than a human. It could produce something radically, radically smarter than a human. We don't know. A lot of people suspect the correct scale would be a logarithmic one, which seems to conform to the pace of improvement being seen, but that's not something we know either.
It could also produce any of the above, but with a reasonably human architecture to the resulting intelligence, or an architecture radically divergent from ours. Although which of those is "better" or "worse" is a rather open question.
This is why AI companies use research in a deterministic and rigorous field (mathematics) as a benchmark - it creates the impression that humanity will achieve comparable progress in other fields.
Having said like, like for other fields (e.g. graphics), hardware capacity slowly increases, and new algorithms are devised, so there will hardly be a strict plateau.
I'm not implying that research progress will be necessarily very slow, but that it's severely misrepresented for sure.
This is no different. Its like a single ant trying to redefine what a ant is. Typically with catastrophic results.