The phrase that Bill Mitchell uses is, "so long as people accept the US dollar, ...", without delving into how long that might be and under what terms.

Which is true as far as it goes, and that is what you want from a theory on an aspect of the economy. After all, the problem with the underlying theory that Krugman uses is that its a closed model independent of money, and so the premise that money is neutral over the long term is built into the underlying logic of the model, entirely immune to falsification by empirical evidence. Grand Theory of Everything economic models have to date ended up being Grand Theory of Nothing In Particular models, running on false equivalences between the terms of the model and the actual phenomena observed in the real world.

I've been accused of being a Marxist, yet while Harpo's my favourite, it's Groucho I'm always quoting. Odd, that.