The more egalitarian a society the more it makes progress.
That was proven after the black plague in Europe - societies that continued to suppress surfs even after the labor force shrank made no progress. See the book Why Nations Fail.
Furthermore high inequality societies are the most unstable and likely to crumble into anarchy. See the book Goliath's Curse.
Now on your premise that a completely egalitarian society could not raise people above manual labor enough to do anything. I'm not certain that was ever true but even if it was it doesn't mean it is true now.
Advances like indoor plumbing under inequality assume the rich want something and are willing to invest in it and make it happen. However these days the rich don't fund squat. Centralized governments have completely replaced whatever purpose the ultra wealthy ever had.
Meritocracy isn't rewarding scientists. The flaw in your argument is that you haven't thought or read anything about this subject and are just spouting the West's actual religious doctrine - you're a zealot.