Whether a true 'generalist' AI is possible in the forseeable future is debatable.
> they can do AI research as good or better than human researchers.
Now your AI is not just a 'generalist' but rather a specialist in AI. A very big leap of faith has occurred here. This also presumes that the AIs are even capable of effective invention and improvisation rather than mimicry and optimization (the only two features we have seen from the very best of cutting-edge ML work so far.)
> And they can make AIs that are even better, which in turn can make even better AIs, and so on.
All of which is predicated on the limiting factor being software and not hardware. If it is the latter then these postulated AIs are hitting the same brick wall as humans and thinking about the problem faster or harder does not magically make the necessary hardware appear.