> Economic transactions create a tax debt which creates demand for currency to pay the government. Sell stock, car, or some Twinkies, even in a different currency, and they get a cut.
That is absolutely correct. But nowhere does it demand that you conduct these transactions in USD. USD ends up getting used because it's convenient, which is because people have to pay their tax debts at the end of the year. However, that is a problem that can be trivially solved by software now, especially as society becomes more and more cashless.
As long as the US government accepts payment for taxes in USD, there will always be some demand for it. However, there is nothing actually stopping people from using other currencies for their normal transactions, which would substantially decrease demand for USD, and thereby reduce its value.
There is no sense in which the US government 'backs' dollars, except insofar as they are incentivized not to do anything to actively destroy them.