You're mixing the concepts of realized and un-realized profit. A market price change marks your open position and gives you an unrealized (loss)gain. But to realize that you have to trade, plain and simple. And someone must take the other side of that.

A more compelling argument against zero-sum is that traders and investors have different time frames and objectives. And, indeed, the classic argument in the futures market is that a farmer who buys a hedge from a speculator represents a potential positive outcome for both.