As Mohenjo-daro prospered, the gap between its richest and poorest households actually shrank. Its shared infrastructure, fair trade systems, and lack of powerful royal elites suggest that equality may have helped fuel the ancient city’s success.
we don’t have a complete absence of evidence, we have some evidence and it leans in an egalitarian direction.
Ok, sure.
Civilization is not a moral judgment, or at least it shouldn't be, although people have historically used it in such a way. In reality, civilization is a political and economic condition or state of development. Morality doesn't come into the picture at all. Neolithic societies were objectively less civilized than bronze age societies, because their economic and political structures were more rudimentary.
Unfortunately, although I don't necessarily want to, I have to disagree that hierarchy is not natural or necessary in human societies. I've always wanted to believe in egalitarianism and fight against hierarchy, but the vast majority of evidence indicates that human societies have always been hierarchical to some extent, and indeed that they cannot function without some form of hierarchy. Hierarchical societies likely predate civilization and the human species entirely. Chimpanzee troops, and the societies of nearly any given social mammalian species, are hierarchical as well, although obviously on a much smaller scale than civilized human societies.
It's perhaps possible to make the argument that hierarchies are not necessary under certain conditions, but to argue that they are not natural is clearly and obviously wrong.