I hadn't thought of that. But wouldn't they all have to agree on a specific crypto, and a specific medium of exchange?
See, that's one of the problems with the liquidity of cryptocurrencies: they're not easily interchangeable, and they're not easily transferable. For instance, if I'm holding 13,000 SMT, and I want to pay you 20 of those, but your wallet doesn't take them, then I can't send money to you. If you're holding BTC and want to try and send them back to me, the same is true (and let's not even get started on the insane transaction fees when using BTC).