>> “Won’t they just replace one fiat currency with another?” <<
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That has only been done once before, with the Euro, and it was not broadly accepted at first. Now that the Euro is in a nose dive, it is unlikely that anyone would willingly accept a new fiat currency.
It will mean asymptotically increased crime and violence, as evidenced by the rioting in Greece. My guess is that the productive portion of the population will demand hard money or commodities as exchange for labor or goods, and refusing to accept any paper instrument of any kind.
That is assuming they have any say in the matter. When the jack booted government thugs point guns at you and say they are taking your produce and giving you government script in exchange, things are different.
In a severe situation, that scenario is probable, not simply possible. A full fledged collapse will put most of us in a war like environment.
So, some of us are well armed. Fine. But be realistic about it. We won't be able to stop them. The best one may hope for is that you take a good number of them with you when you go.