I came to a similar conclusion: The sole criterion for the viability of a currency is acceptance in trade.

This simple truism explains every currency that "works" and every one that didn't all at once.

It can be a terrible currency, but if it's generally accepted in trade, too bad. Governments can mandate it, but that won't always save it. Just look at Zimbabwe dollars. They were official. And all but worthless. USD is accepted worldwide, even though its based on less than even a shitcoin, and 90% of it doesn't physically exist according to the Fed themselves.