good point; zero-sum only if you consider btc itself as having no value.
BTC is worth what I can get for it. What I can get for it is directly related to it's USD value. It is a zero sum game. Someone has to pay.
In a FOREX sense; trading BTC/USD currency pair against a counterparty, yes.
Lower volatility will come with market depth.
Organic price appreciation will come with adoption (transactions)