Actually, I didn't say that human nature under no circumstances will change or can be mitigated in a way... I said quite the contrary. What I did say is that we need external coercion or constraints that would make us live in an anarchical way. The reason for this (as I see it) boils down to a competitive edge that hierarchical societies have before theoretical non-hierarchical (since there are none as much as social beings are concerned) in the struggle for existence...
I wonder how that differs from what I said in practice, given that you explicitly stated that without external coercion or constraints, an anarchical society would give rise to a hierarchical one; but fine, I won't pursue that. Instead, let me try and address the rationale behind your conclusions and maybe we can get somewhere with this.
So, your point is, hierarchical societies have advantages over anarchical ones, so that even if we started with an unconstrained non-hierarchical society, it would eventually turn to a hierarchical one; now, to account for this, you only mentioned the fact that hierarchical societies exist and that there are no large scale, technologically advanced anarchical societies to speak off at this point in time - an advantage in itself, of course. I'll try and address this point then, but feel free to detail other advantages you feel they might have.
You are of course right in that they don't exist at present; but it doesn't necessarily follow that hierarchical societies must have a natural advantage over non-hierarchical ones because of this. I'm sure this argument must have been used before the first parliament was ever created, before republics were established, or before people were able to vote either for their representatives, or directly in referendums to decide what measures they would like to see implemented in society. Established power has always resisted change, but changes are always taking place.
It's as I said in previous posts: this line of thinking tends to ignore a great deal of reality. Even relatively small successes like Catalonia and Aragon, during the civil war, took decades of education and experiments before breaking through; these things don't happen in a day. And everything considered, even what we call democracy (a far cry from what it should be, as we know, but still) hasn't been applied for that long (a couple of centuries), compared to millennia of totalitarian forms of government. And of course the basic idea behind democracy has been known for a long time, but it took this long for conditions (education, opportunity, etc.) to allow it to be implemented. Still, would you go back to the 15th century and say such a thing would never work on account of nothing like that existing for any known large scale society?
Another example I mentioned earlier was slaves. Look back and you'll see arguments that are not all that different: "our society can't exist without slaves; if they don't work the fields for us, we'll all starve", or some equally self-serving argument. Yet society wasn't worse off without slaves (you can argue why that is, but that won't really change the point). For women, one of the arguments against giving them the vote was that it would just give a second vote to the husband (so, being unfair to unmarried men), because women obviously couldn't deal with stuff like that. Turns out they could, and society is better for it (and more equal).
People are increasingly more involved with the societies they are part of, and as I'd said, for me the tendency seems to be positive and moving towards greater equality. In this sense, the reason you don't see large scale non-hierarchical societies is not because hierarchy naturally confers some advantage, but simply because we're not there yet; a lot more education, experience and opportunities are needed before they can exist.