> But how can you conclude that such a society won't progress in isolation?
Don't know. We aren't bees or ants - we're tribal hierarchical apes - so we don't have a lot of isolated (or even positive) "everyone is equal here" examples to study.
The socio-biologist E.O. Wilson, when asked about such altruistic-sounding theories, responded: "Right theory - wrong species".
This is false, or at least unjustified. Sure, an equal society may not grow as fast as an unequal one, and so the latter may take the former out of the competition. But how can you conclude that such a society won't progress in isolation?