So sad, so true. This is what happens when you play a zero-sum game. Even if you win, someone has to lose.
When you play football too, if you win someone has to lose. And yet there are positive externalities, to wit millions of people watching the World Cup.
In trading, the externalities might not be so obvious but they do exist.
A short answer would be: one of the externalities that traders like the OP provide is lowering transaction costs for other traders such as investors. Compare trading stocks with speculating in real estate: buying a $150k house and selling it later might involve (say) $10k worth of commissions to your real estate agent, while trading fees on a comparable stock transaction are usually much lower.
Please see my other comment below and (especially) the linked article.
Another positive externality of trading is lowering the cost of funding for enterprises. Companies go through a lot of trouble to make an IPO; they do so because getting listed increases their sources of funding.
A longer answer would probably require a blog post, so that we could discuss whether I'm "cheating" -- after all don't the points above boil down to providing liquidity? Am I really "cheating" or maybe your condition is unreasonable? Et cetera. If I write such a post I'll post it to HN, though I doubt I could do better than Larry Harris.
I'm not interested in your bonus points, but in the correct answer. I believe this is it: Large, liquid markets reduce economic friction. They make an economy larger, more diverse, and more adaptable. The low friction allows small changes in supply and demand to quickly propagate long distances, producing responses where otherwise the information would have been lost to the friction. You don't have to look far in space or time to find places where this "zero-sum game" isn't or wasn't available. Just look for poverty and lack of social mobility.
There is an economic progression from the switch from barter to money, to lending of money, to spread of risk by enabling minority ownership of multiple ventures (early corporations), to easier transferal of ownership in smaller pieces (allowing more of society to participate), to large-scale, low-friction, highly-liquid markets used by everyone, directly or indirectly. Each step further reduces economic friction, which lowers the threshold for economic activity to take place, causing more of it to occur, increasing the prosperity of the whole society.
Even if you lose money personally in the "zero-sum game" with unfortunate investments, you're still probably losing only a portion of the money you made working at a company that wouldn't exist if not for large, liquid, low-friction markets.
When you play football too, if you win someone has to lose. And yet there are positive externalities, to wit millions of people watching the World Cup.
In trading, the externalities might not be so obvious but they do exist.