“We have never seen Rothbardian (or Friedmanian for that matter) anarcho-capitalism in the world before now. Why is that?”
I speculate the origin of the state is tied to the origin of religion. Religion provided explanation and was used to justify authority. I speculate bad ideas easily became dominant with the state’s creation. Its existence eventually made it considered normal, a part of life, although the reason it allegedly needed to exist was never logically proven. Even in the 21st century, U.S. presidents place their hands on the bible when promising to adhere to the U.S. constitution.
“The earliest states formed as people voluntarily sought protection against this predation.”
Governments are not voluntary organizations, therefore people could not agree to voluntarily join them.
“Therefore, in the past, anarcho-capitalism wasn’t viable, since it never made a significant dent and was not able to survive in a world surrounded by aggressive states and barbarians.”
This reasoning is anything that did not occur in the past could not have occurred in the past.
“In the first half of the 20th century, and this is just a hunchism on my part, but I believe that if the USA had already been anarcho-capitalist and without nuclear weapons”
If the U.S. was anarcho-capitalistic, World War I, the Great Depression, World War II, and the Cold War may not have occurred. Even during those events resolving them could have been done anarcho-capitalistically. For a longer explanation view Chapter 6 in Viception.
“How can anarcho-capitalism acquire the necessary power to defend itself against external predation and threats in today’s world?”
Government does not guarantee safety. Governments can lose wars. Governments can kill their own citizens. The technology for defense is the same with or without government. Under anarcho-capitalism, defense is for that purpose: defense, not offense.
“One flaw I find with the PDA superior efficiency argument is that if it were true, states would never have formed in the first place.”
If FexEx and United Parcel Service (UPS) were more efficient, then the United States Postal Service (USPS) would “never have formed in the first place.” Wrong.