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.