Where does the trust come from for transacting in dollars, or Spanish pieces of eight? I'm not saying the monero economy is likely but there's absolutely no reason why it couldn't theoretically happen
Courts, law enforcement and contract law. All of which will take a dim view of using a currency which appears designed wholly to make their function harder.
I think crypto already exists as a parallel economy. Especially between certain states.
You use monero not to exchange for fiat but, for example, oil.