unequal society was sustainable exactly until now. there is no evidence whatsoever that it won't still be.
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.
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.