> But governments can be poorly run so I suppose “stable” is not the right word.

no, but the currency belonging to a particular gov't is as stable as the gov't itself, so your statement is still true; vis a vis the Zimbabwe currency vs the US dollar.

An ad-hoc system of local warlords who take power isn't more stable, even with a blockchain.

What people refuse to believe is that a currency is actually a reflection of the real world. Yes there is some filtering going on and time lags can be huge but any well managed currency is only going to perform as well as the real economy.