BUT indeed I believe that anarchy, no matter how it starts, is unsustainable, and is quickly replaced by a government, internal or external; and, in the first case, often an authoritarian government, rather than a democratic one.
I think you just tried to redefine your position to something different from all the posts you made before that one here - but even if that is what you believe, then you still don't get the actual meaning of 'anarchy' as most anarchists use that word. Anarchy is an intentional state that is achieved, not something that just 'happens when a government collapses, etc.'.
As far as the sustainability of an anarchic political equilibrium goes, you may well be right. I have never seen any state reach a state of anarchy, as I understand it. Communist governments ostensibly aspire to it, but I think it's clear that none has achieved any such thing so far - and I have never seen any nation with what I would consider a truly libertarian government, let alone an anarcho-libertarian one.