By that definition, which financial instruments aren't negative sum? E.g. stocks trade hands in a very similar way, and companies can always issue new shares.
The same thing could happen with crypto. A smart contract developer sells crypto to fund development of a smart contract. That smart contract can provide new utility and drive demand for the currency.
The money builds a new factory. Or in the case of Google, a data center, or other such item that allows the company to make more money.
Later, when the company is worth more, the shareholder sells the stock back and/or gets a dividend (aka, a slice of the profits).
Everyone wins.
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In contrast, the BTC you bought was likely made using stolen electricity. So we're already negative sum to the start.