In an-cap society you pay a defence agency to stop crime in places where you visit, not everywhere in the state, so I imagine that there will be lots of unsafe places and lots of really safe places. I think many people wouldn’t like that and would prefer to create a territorial monopoly in which everyone who comes have to pay for protection of that entire territory. For example towns and even cities may have such residency agreement. But doesn’t it mean small states will develop and we will become a city-state statist society?
That is one possibility. And these cities/neighborhoods may be indistinguishable from states, but it would still be a relatively decentralized order.
But if we assume that this is a realistic possibility, then we can also assume that an entire state will also be a realistic possibility, because the same externalities that result from police protection also appear in provision of national defence. So the incentives will be similar, to create states in order to internalize these externalities.
A stateless order lapsing back into statism is a realistic possibility, yes.
And what are the reasons you think are likely to cause it?
The social order is far too complex for us to foresee its development. Perhaps the statist order as it exists presently is in some (still unknown) fashion superior to anarchy, and thus would naturally emerge anew form anarchy. We’ll never know until we try. Until we do, there’s no point in making much of a fuss about it. After al, if it really happens that the statist order re-emerges from anarchy, than it is indeed superior.
I’m becoming more and more convinced that city states or neighborhood states will be likely. There is simply no way private defense agencies will be able to make money by patrolling huge areas. There is an enormous externality in this. So the difference between an-cap society and minarchy will not be very significant, it will be small states vs. big states. Why then it is so difficult to convince minarchists that an-cap is possible?
When you have to go to a different area than the one that your PDA covers you buy a temporary insurance policy from some local company.
Statist orders also collapse all the time, usually to form other statist orders that collapse. I don’t know if it is a question of superiority, but that it’s impossible to have any permanence and silly to expect it.
It could happen that ‘externalities’ are indeed significant enough for a single individual to own a block or city, and then rent space to tenants. There you have a fully moral ‘state’.
Fair enough, but statist orders that emerge of the ashes of old ones are different. You cannot say that the same system is re-enacted. Social patterns evolve; the state has evolved too from feudalism, in turn evolved form tribalism.
Or humans are too stupid to live peacefully with each other 
You’re thinking about anarchy through the lens of central planning. If we knew the best possible way to provide defense, we wouldn’t need an anarchic order in the first place.
