I'm replying without having read the entirety of the text you've referred to by Proudhon, but it looks interesting—thanks.

Some raw thoughts of mine if I may (feel free to add seasoning):

You mention that capitalism is definitionally zero-sum, and you seem to be facing quite a bit of resistance. I've had similar thoughts (perhaps still premature) that capitalism is zero-sum, but only (?) under a strong definition of "zero". I've not fleshed out my thoughts completely, but I suspect there are intangible/abstract dimensions along which we maintain some kind of equilibrium, regardless of what we do. "Do" here is quite abstract, but as a first approximation in the realm of economics, it might refer to any act of investment, compensation, or labour. (I may be abusing some technical terms in economics here—not my home turf.) A separate question could then emerge as to how significant these intangible/abstract dimensions are.

Actually, I'm not even sure that this is specific to the context of capitalism. However, whether something is a zero-sum game would seem relevant to systems obsessed with objective quantification, and where that quantification is heavily involved in steering moral views (or decision making), and I view capitalism as one of them.

It’s definitely not specific to the context of capitalism

capitalism however makes transactionalism the explicit structure such that it cannot coexist with any other type of ownership regime by function

That is to say, if you look at anarcho socialist philosophy it can theoretically coexist with other philosophies inside the same state and action space

Historically however, we have not found a stable equilibrium for the lived reality of our experience such that we could map it cleanly onto some discreet and identified philosophical framework

So neither anarcho-socialism nor capitalism is a sustainable equilibrium point due to the constraints of a human biological substrate

Claiming that “it could” or “can” or “is the best we can do” are all beside the point, because they ignore the intractable fundamental fact of separating human systems from all other systems

Every possible game is zero sum because the universe isn’t creating more matter or energy, it’s just moving around. How we move it around is the problem to solve and anyone using weak justifications with bankrupt epistemological foundations is just wasting everyone’s time.