Agreed. Historically speaking it's akin to have the Mongol hordes appear on the horizon and grind you into dust. Roving despotic bands would rule the day in any true anarchy scenario. Anyone that disputes that disputes thousands of years of human history.
You do realize that neither the Mongols nor the peoples they conquered were anarchists, don't you? But even setting that aside ...
History is contingent upon a vast, vast, VAST arrays of inter-related variables.
Change any one of those variable even slightly and "history" could have been extremely different from what it actually was.
And many of those variables (such as geographical availability of resources or miltary tactics & technology, to name only two of thousands upon thousands) have
nothing whatsoever to do with abstract political and philosophical principles.
It's fun to discuss theoretically. 300 million isn't that shocking given the exponential population growth within the last century coupled with technological advances in the art of killing. Some estimate the Mongols wrecked death upon 40 million during their reign. History is on my side, someone will wear the reigns whether by government decree or will to power.
So (by your assessment) we are screwed no matter what. "History proves it." So why even bother?
History is
not on your side - nor is it on anyone else's side. History "proves" absolutely
nothing about what is or is not possible.
At best, history merely reveals what did or did not happen "once upon a time" (under the particular conditions and circumstances of that time).
History is composed of
a posteriori contingencies, not
a priori necessities.
There was once a point in time at which you could have used history to "prove" that constitutional republics are not possible - because none had ever existed before.
In fact, you could even use modern history and current events to "prove" that right now, today, constitutional republicanism is a complete failure.
And anyone who did so would be just as full of crap as those who claim that history "proves" that anarchism is doomed to fail.
(The only "argument" against anarchism that is even lamer than the "historical" argument is the "people are bad (so we need some people to be in charge of everyone else)" argument.)