Not GP author. I'd like to continue the conversation though. However, be warned that my view is actually closer to reducing the economy to thermodynamics. I don't intend to overturn every single point you've made, but I hope this doesn't preclude a productive discussion.
> Services are an extremely clear example of positive sum - no resources disappeared from the world, as much money was gained as was spent, but on top of it somebody got something of value.
I think it's very hard to fall back on services being positive-sum on a gross basis (i.e., 0 inputs, positive outputs) to justify that it is positive-sum on a net basis.
What kinds of services actually consume no resources? I could agree that, in isolation and on a marginal basis, a particular exchange of services for money might deplete a negligible amount of (physical) resources, but when you consider the operation of the entire industry (supposing a mature industry, i.e., that there is an industry to speak of), can it really be said that the entire industry consumes no resources? A prototypical counterexample is any service that relies on physical equipment: I would view that physical equipment always incurs wear and tear, and this is potentially substantial for sufficiently large industries. The wider umbrella here are all the other various externalities of the service.
(A good rebuttal to the physical equipment counterexample is actually where we've mastered the materials science well enough that, miraculously, the wear and tear outlasts the lifetime of anyone involved and hence where the equipment feels impervious to wear and tear... I resort to time horizons, which is another aspect of "scale". Something like GDP [growth] tries to normalise for time scales, but sadly I see this as falling prey to the same shortcomings as any kind of prediction activity.)
Personally, I consider it reductionist to try and measure every transaction with a currency value and then aggregate for a GDP. (The next key phrase in this train of thought is "Goodhart's law", which happily also gets addressed in the OP site [0].) However, I do also appreciate that this is a really fundamental paradigm in modern implementations of capitalism to attempt to uproot.
One way through which I can appreciate that capitalism is non-zero-sum is: across multiple different dimensions/axes/facets of measurement (currency value may be one of them), transactions incentivised by capitalism are not "zero" on all of them simultaneously. Under capitalism, it is that the transaction is positive by currency value which incentivises its own execution.
But there are lots of service industries where an undue focus on the currency value pushes us towards undesirable outcomes (necessarily on some axis besides currency value or GDP). For instance, some services are just innately incompatible with commercialisation. (Arts and culture comes to mind as one. Basic research is another.) When you attempt to offer/conduct these services under capitalism, you invariably need to moderate/regulate/limit the offering due to capital constraints. As in everything, moderation is sensible, so the next question is: are there enough people with enough influence thinking about whether we've gone too far? In a system where garnering influence is highly positively associated with accumulating capital, the answer seems self-fulfilling...
[0]: https://nonzerosum.games/goodhartslaw.html
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EDIT: I just realised that the "G" in "GDP" is "gross", for being gross of depreciation ("wear and tear"). This is a pretty big revelation for me, since it probably sheds some light on why I think GDP gets undue focus. Nevertheless, I think the principle of what I said above still stands.
