I think what we know now about homo sapiens shows that there is plenty of truth about the immutability of human nature. We are prone to irrational urges we can contain only so much. We are still Stone Agers. In fact, it appears some Stone Age societies were much nicer than ours.

For instance, on top of wanting their needs met, they don't want others' needs met more than theirs. And it's been demonstrated that their ability to perceive justice in such situations is warped. ("Thinking Fast and Slow" by Kahnemann.)

In Zakaria's "Age of Revolutions" he refines the critique of human perfectability, talking about the French Revolution, where the government thought to create social justice overnight, and the structures they imposed were too discombobulating for people. Regardless of their support for the government's goals, they couldn't cope with the implementation, the changes to institute justice.