The assumption is that people who are free from coercive hierarchies are powerless to act in their own self defense, alone or in cooperation with each other.

That's the problem. Acting alone is not an option since an organized force will always defeat a disorganized one, and "cooperation" would mean forming the same kinds of hierarchies and governments that we already have, except they won't have the centuries of stress-testing our present democratic systems have undergone and will therefore fall to corruption and authoritarianism much more easily.

This is why anarchism is such a bankrupt ideology. At best it consists of people willing to burn the world down to institute a system that would be the same but worse.