Forex markets are basically as liquid as you can get -- of course some exceptions, but I think the idea still holds.
While an existing fiat currency might derive its confidence from different sources than that of a digital currency (taxes, military, etc. vs. blockchain, mining, contracts, etc.), it does not make them that different. People still declare the coins value on that confidence/demand.
Yes, definitely on the forex thing. But with cryptocurrency-level liquidity there is no reason for any normal consumer to hold funds in any particular crypto currency (or any at all), since they can exchange them cheaply and quickly on demand. That is, unless they're speculating it will increase. But this increase presumes scarcity. And so the question is: whence scarcity?
While an existing fiat currency might derive its confidence from different sources than that of a digital currency (taxes, military, etc. vs. blockchain, mining, contracts, etc.), it does not make them that different. People still declare the coins value on that confidence/demand.