> But I would argue the world is not zero-sum. Modern life for middle class or poor class people is a lot better than the vast majority of history
Zero sum is not a binary. For things where we are able to increase output using technological progress to the point where supply sufficiently meets demand, sure, it doesn't feel zero sum. Nobody is arguing that we need to fix that -- the challenge is actually preserving the situation in these cases (it's bad to kill the reward signals that make the economic engine go).
For things where we cannot rapidly increase output, things that have unlimited demand but very limited supply, the rich are likely to monopolize. In those cases, it is zero sum. Public spaces (think of the beach access controversies with people like Vinod Khosla), farmland, housing, fresh water, college admissions. Housing and land in particular are really tough.
Strategies like wealth redistribution (but _also_ other related policies!!) are needed to mitigate the latter.
Wealth centralization is also anathema to democracy -- just ask proof of stake folks. When the wealthy get to make all the laws, you won't want to live in that society.