I'd put it is that conducting international trade in a single currency is more systemically stable. Iow, the currency's ubiquity and total amount buffer shocks

I'd put it is that conducting international trade in a single currency is more systemically stable. Iow, the currency's ubiquity and total amount buffer shocks to the system, in a way that multi-polar currency use could not tolerate. If some folks decide to break {insert smaller international currency here} because they think it's overvalued and they don't use it, they probably have the means. But it's difficult to tip over a world-economy sized apple cart. And a single trade currency also mostly keeps nation-states out of the currency warfare game, because it's a gun pointed back at their head too.