Earlier quoted context omitted.
Indeed, though states have boots and guns on the ground to a last resort backup the value of their currency (including protecting against competition). So I'd pick "Fed-coin" as the last one to go down.
I've been reading up lately on how fiat money systems work, and it doesn't seem to me that boots and guns really help. Certainly there have been countries with plenty of soldiers who failed to maintain the value of their currency. You have to pay taxes in the local currency, but if the currency crashes you just have to pay a higher nominal amount of taxes. You can outlaw competing currencies, but you can only do that…
It only has value because a government can ask others to settle part of this debt or take their property otherwise. Naturally, the amount of debt a government can issue this way is limited by the value of property it can threaten.
Zimbabwe, for example, cannot reach much of valuable property despite having some guns and boots. But the USA is a very different matter.