> It doesn't scale and so isn't really workable in the modern world where we need systems that account for millions of people at a time.

The thing about horizontal power structures is that, unlike hierarchical structures, they don't scale. Scaling happens when a system grows and it becomes more difficult to centrally plan and manage. Natural systems like the universe itself can be as small or large as they like for this reason. Same with animal populations. Of course such a manmade system wouldn't resemble what we have now, but that doesn't mean that such a thing can't exist. Hierarchies are the exception in nature, not the norm. To frame horizontal structures as unrealistic is to claim that reality is unrealistic.

> Science is a great example of something a tribal communist/anarchist society could not have. Science requires allowing for specialization, which means one less person working towards the survival of the group.

You mean like the specialization of Aristotle, who did philosophy, physics, metaphysics, and other subjects? Or do you mean the mathematician, philosopher, and scientist that was Isaac Newton? Perhaps you are referring to the specialization of Albert Einstein, who was a philosopher, a mathematician, and a scientist. Also a socialist.

The greatest contributors to the advancement of human knowledge rarely resemble the hyper-specialist we see under capitalism and more closely resemble the holistic thinkers of broadly talented people like Leonardo deVinci. Some degree of specialization is necessary, even in the primitive societies you're referring to, but too much specialization and there are no longer any advancements.

In fact, the factory method of industry was in part designed to keep workers so specialized that they'd never command a better wage or grow into potential competition. Like anything, specialization is not always beneficial at all levels of extremity.

> Communism is impossible, not because every society who's tried it failed, it's impossible because it fails to account for human nature. Communism can only work if everyone always buys into the shared collective efforts of the group. But there will always be people who seek to take advantage of situations to gain advantages for themselves over the group, which is exactly what happened every single time it's been tried.

This is based on the idea that because you observe capitalist behavior under capitalism that this means that capitalist behavior is "human nature". It's a common fallacy to assume that your era or culture represents the sum total of human capacity. In reality, communism represents one potential path among many that people can take. A great example of everyday communism is the household. Some households are feudal in structure where the income earner controls everything and dictates everything. More commonly, though, families in a household pool their resources and share for the benefit of the family. This is especially true in poor families that cannot afford the more authoritarian alternative. The fact that both of these kinds of households exist, though, demonstrates the amazing plasticity of so-called "human nature".

> It gives ownership of capital to those who work it rather than those who own it.

The workers use what they own and keep what they produce, the factory is no longer capital (someone else's private property) and the product is no longer capital (someone else's private property). Communism as defined by Marx takes this a step further and removes money and the state as well.

> An interesting thought experiment perhaps, but certainly nothing to actually take seriously. Unlike environmentalism and feminism, which can and do exist.

And criticism from the right of feminism is that it's a utopian fantasy about putting women in charge. Criticism from the same towards environmentalism is that it's a utopian fantasy that nature above humans. How is your critique different from this? Ultimately not. Like with the others, it's born from a misunderstanding of the subject.

All of these are frameworks which approach problems from a particular lens. Feminism from power relations between men and women. Environmentalism from power relations between humans and non-humans. Marxism from power relations between owners and workers. Anarchism from power relations between the government and the governed. None of these are proposed utopias.