What will really happen is that state-issued currencies will not completely disappear.

They will become hyperinflationary, and that will be the new normal. They will still be used to pay taxes and denominate public debt, but they will be deeply inflationary due to the State’s financial situation:

No State has the ability to pay its infinite debts to banks and its own people without turning on the printing press (and in the process destroying its currency, in which the debt is denominated, thereby reducing the real value of the debt).

They will be far, far less relevant, but at least for the next 50 years they will not cease to exist.