What secures private property?

the most common objection voiced by the economically literate is that anarcho-capitalism would quickly decay into monopoly, whether through war or merger. And it's a short step from a monopoly defense firm back to government.

The standard anarcho-capitalist rebuttal is to compare the scale economies in the market for defense services to demand, and see how many firms the market has room for. If there is only room for three firms, it's plausible that they might merge to monopoly and become the new government. If there is room for ten thousand firms, it's totally implausible. As David Friedman wrote in The Machinery of Freedom:

If there are only two or three agencies in the entire area now covered by the United States, a conspiracy among them may be practical. If there are 10,000, then when any group of them start acting like a government, their customers will hire someone else to protect them against their protectors.
...My own guess is that the number will be nearer 10,000 than 3. If the performance of present-day police forces is any indication, a protection agency protecting as many as one million people is far above optimum size.


But there is a simple rejoinder to Friedman, If scale economies are really this weak, why have states emerged and remained stable for thousands of years?