(My apologies; I didn't fully understand what you were asking for.)

First, it doesn't claim that philosophy makes zero progress, just that science/AI research/etc. makes more. There were still broad swathes of knowledge (e.g. linguistics and psychology) that split off relatively late from philosophy, and in which philosophers were still making significant progress right up to the point where the point where they became sciences.

Second, philosophy has either been motivated by or freeriding off of science and math (e.g. to use your example, Frege's development of predicate logic was motivated by his desire to place math on a more secure foundation.) But the main examples (that are generally cited elsewhere, at least) of modern integration or intercourse between philosophy and science/math/AI (e.g. Dennett, Drescher,, Pearl, etc.) have already been considered, so it's reasonable to say that mainstream philosophy probably doesn't have very much more to offer, let alone a "centralized repository of reductionist-grade naturalistic cognitive philosophy" of the sort Yudkowsky et al. are looking for.

Third, the low-hanging fruit would have been taken first; because philosophy doesn't settle points and move on to entire new search spaces, it would get increasingly difficult to find new, unexplored ideas. While they could technically have moved on to explore new ideas anyways, it's more difficult than sticking to established debates, feels awkward, and often leads people to start studying things not considered part of philosophy (e.g. Noam Chomsky or, to an extent, Alonzo Church.) Therefore, innovation/research would slow down as time went on. (And where philosophers have been willing to go out ahead and do completely original thinking, even where they're not very influenced by science, LW has seemed to integrate their thinking; e.g. Parfit.)

(Btw, I don't think anybody is claiming that all progress in philosophy had stopped; indeed, I explicitly stated that I thought that it hadn't. I've already given four examples above of philosophers doing innovative work useful for LW.)