If you consider the unnecessary death and suffering of other people, and major environment impacts, to be "sustainable" then yes, unequal society has been entirely sustainable.
In practice the inequality has lead to massive unrest (eg civil war), disease, and famine time and again.
You can't honestly look back at history and consider that inequality has been anything other than a cause of immeasurable misery.
from my point of view all those deaths and massive unrests resulted from various groups trying to do away with inequality, usually by taking what's not theirs.
Lenin (for example) didn't walk into a prosperous, free, egalitarian country, and persuade everyone to rise up against the existing system. The inequality, poverty, and misery gave rise to people willing to listen to Lenin, and to try what he was selling.
I'm not excusing Lenin. I'm not apologizing for him. But without the existing inequality, he would have gotten no traction.
he also wouldn't have gotten any traction if people had no concept of envy. yet eliminating envy is just as stupid as eliminating natural differences between individuals.
In practice the inequality has lead to massive unrest (eg civil war), disease, and famine time and again.
You can't honestly look back at history and consider that inequality has been anything other than a cause of immeasurable misery.