A fiat currency backstops its value at least with the issuing government's willingness to accept it for payment of taxes.
What is wrong with current monetary regimes? Sounds like you are suggesting a return to fixed/commodity money regimes which have failed many times over.
As you suggest , fiat currency issuers backstop the value of their currency via taxation. But its not their willingness to accept the backstops it, its the power to impose mandatory taxes on the population that gives value to the currency.
Warren Mosler -one of the founders of Modern Monetary Theory-
has a great allegory to describe how it all works: ( I paraphrase) A group of people are in a room and I have business cards in my pocket. I have a gun in my hand and no one else does and I am standing in front of the only exit. Now I decree, in order to exit the room it will cost you 1 business card. Suddenly, the business cards have value and folks in the room will provide me goods and services in exchange for payment in business cards.
