2/2
First, we need to figure out what could drive political will to enact or strengthen hierarchies. And on this point I might be considered a silly dogmatic, but I actually tend to fall into the Marxist pit of all materialism everything. Despite being a libsoc, my thought is still shaped a lot by Marx.
In my opinion, most situations of populations "willfully" subjugating or enacting hierarchical oppression both historically and hypothetically can be attributed to material conditions, or class conflict.
The fiction of an outside threat to your society pressing you to give up liberties to the state results from the existence of the state, meaning that there is a conflict of interests between the state and the people.
The other fiction mentioned in this thread, that of an outside thread pressing your material conditions, also results from the fact that people are alienated from their labor.
If there is an "outside threat" that threatens your specific material life, does it not hold up to even the tiny bit of scrutiny that most Holla Forums-nazis grant their thought, that for this to make just a tiny bit of sense, somebody has to benefit from this subjugation of you.
Like, Jews are evil, look at how much money and control they have! Niggers are dumb and stupid, look at how little money and control they have!
There are probably other causes for this political will in class society, but I'm not writing a fucking book so fuck you, these are the ones I see as most pertinent.
Here I'm of course talking about true world anarchism, since otherwise there truly would be these class conflicts between anarchist society and repressive society. But on the other hand it's silly to talk about "regression to hierarchical society" when we're still living in hierarchical society.
So okay, let's just presume that I'm wrong here and that political will for repression arises in some ancom society, despite there being no material basis in the conflict. There's a will to establish hierarchy, despite previous eradication of this hierarchy. There are then two situations that lead to the establishment of hierarchy; that of a political minority who would benefit from hierarchy seeking establishment of hierarchy establishing with the help of self-subjugating minority of the larger majority of to-be opressed, that of a political majority who would benefit from the subjugation choosing to subjugate a political minority.
For the first case to happen it requires there to be a conflict between those to be subjugated, for some to refuse to stand in solidarity with the others, which I think is a very unlikely scenario. It might happen often today, but I think libertarian shapes of governance and culture will inherently help combat this.
In the second case, it would be a hierarchy just like that of the slave society (not in it's oppressiveness, but in the exploiter-exploited relationship).
In this case (and the first as well, should it happen), my opinion is that the rights of individuals to not be oppressed are more important that the rights to oppress, and I personally think that we ought fight to eliminate that oppression - no matter how it was enacted - and would push to end that oppression, violently if needed.
Finally, if you're actually talking about a near-future post-private property society, what would actually happen is a lot more simple. There would probably be a limited acceptance of their political will, so long as they don't start murdering people. People who feel repressed would either leave or fight against repression, and finally due to material conditions improving and no serious outside threats to security, society would socially progress not unlike modern society, it would just be a hierarchy just like any other cultural hierarchy already in place. Hegelian dialectics baby.
Basically, it boils down to two core facts that most socialists agree on, libertarian or not, that oppression generally results from material hierarchy (such as that of the capitalist and worker), and that (independent of the first fact) the absence of material hierarchy positively supports (Or more radically, enables) the fight to end non-material hierarchy.
That's my thoughts.