Financial markets are positive-sum.

Not necessarily. First, it depends on the market. Some are zero-sum, and about others one can say that they are NOT zero-sum, but that's it. They might be negative-sum or positive-sum, depending on the circumstances.

If you just buy a bunch of stocks and hold onto them, on average you'll outperform cash.

That also depends. Average over what? Which countries and what time periods?