All societies have ended due to a combination of internal and outside factors. That is the test of a society. To claim that anarchist societies do not have internal issues is intellectually dishonest. All capitalist societies that have failed have done so due to outside factors. So too have all monarchist and communist if the true believers are to be listened to. When the USSR and the CCP started it was economically anarchist so I do not accept the claim that all anarchist systems ended due to outside factors. They also naturally consolidate.
The key difference is how much suffering is caused along the way. Anarchist society ascends into fascism so easily because it already controls the resources and is a single-party system that does not allow dissent. All the elements are there minus the natural rhetoric and for people to vote themselves a better position (tyranny of the majority). You cannot stop that. It is going to happen because it is human nature.
I honestly don’t expect to change your mind in this conversation. That’s not how it works. I was once a radical thinker. I do hope that I’ve instilled some nuggets of thought that eventually turn into a worldview that is less likely to cause harm. Very few radicals that actually implement change survive it and they tend to cause a lot of pain along the way. The rest end up in teaching.