You argument is not straightforward. It is logically flawed.
> The universe is much larger than our appetites, and that's the horizon we're talking about with economics- centuries, not the heat death of the universe.
You don't have access to the universe. You only have access to the earth. If you reduce the time horizon to centuries you must also reduce the resource horizon to that which is available on earth. The earth is a highly limited resource. The situation is not even close to your flawed argument of limited appetites vs unlimited resources.
That is fairly straight forward. I never talked about the heat death of the universe nor any epic cataclysms outside of the human experience. That came from you. Thermodynamics applies to smaller systems as well as the universe and is highly relevant to the human experience.
>Your comment about gasoline is a good example of a resource that has a real world constraint, but it doesn't show that economics is zero sum. It shows that the limited game, where the goal is to obtain as much gasoline as possible, is zero sum. If I also use nuclear power, I'm not playing that game.
How is nuclear power not a zero sum game? Follow the economic chain. Nuclear power involves a resource called uranium. Uranium is used up in process of generating nuclear energy. Uranium is mined from the earth and therefore limited. Additionally, the global supply of uranium is nowhere near high enough to be classified as a seemingly 'unlimited' resource. You cannot escape the game.
Your best argument here is the sun. The sun can be classified as a seemingly 'unlimited' resource. However the bottleneck here is the amount of solar energy we can extract from the sun. That energy amount extracted is limited by technology and physics to a concrete amount. That concrete amount is significantly less than the energy we demand.
>We have a sad tendency to ignore negative externalities, as a species. And it could be our undoing. But countering that by claiming that human econonic activity is zero sum is an abuse of game theory.
By claiming that economic activity is not a zero sum game is an abuse of reality. Find an example that isn't zero sum. Find a resource that isn't constrained.