I think you'll find that FIAT currency is exactly what you explain with your post. Since all countries decided to move away from the gold standard pretty much every currency is realistically worthless.
When financial corporation, such as a bank, grants a loan to a customer, and thus creates new money, the loan contract is shown as an increase in assets: the bank now has an additional claim on debtors — this is the borrower's legal obligation to repay the loan (which is secured by the collateral of specified movable or immovable property). But, together with the increase in assets, the newly created money appears as a positive entry on the liability side of the bank's balance sheet. IOWs, whenever some firm or person receives the newly created money from the borrower's bank account, the value of this money comes from the fact that it represents the bank's legal liability which is backed by borrower's legal obligation to repay the loan. So, when banks create money, this money is backed by either - legally enforceable obligation of borrowers to repay their loans or the collateral of specified movable or immovable property that a borrower offers as a way for a bank to secure the loan.
That is why, if a situation occurs where nobody wants to accept fiat money in exchange for goods and services, those who received the loans are legally forced to accept it, otherwise they would be unable to settle their obligations to the bank and a legal action would be taken against them or the bank would seize the collateral to recoup its losses. Additionally, the value of fiat money comes from the fact that it is legal tender for all debts, public and private, which is especially important today when countries have high tax-to-GDP ratio.
So saying that fiat money is worthless, simply shows the lack of understanding of how legal, monetary and banking system work.
The only thing which is entirely worthless is Bitcoin. Since Bitcoin is neither legal tender nor backed by any legally enforceable obligation, if a situation occurs where nobody wants to accept Bitcoins in exchange for fiat, goods or services, they can neither, be used to pay taxes nor any legal action can be taken by their owners to get some kind of value out of them. That is why Bitcoin is nothing but a modern ponzi scheme, a collection of empty numbers on the internet that will collapse just as all other ponzi schemes have collapsed.