What a strange opinion you hold!
| All currencies are backed by our willingness to accept them...
This is true!
| ... that is the only backing that any currency ever needs
This is clearly false- just consider any currency that has experienced hyperinflation. Currencies backed by commodities are not subject to such troubles. Or consider the hidden tax of even nominal inflation caused by monetizing debt (ie, printing "mind backed" currency to pay debts). Such systems are essentially a transfer of wealth from the poor to the rich since poor people don't have access to financing/loans.
| So why would they take a fiction over a real shirt that that gives them real value?
Of course both the shirt and paper money have value, subject to supply and demand. But, the paper money tends to hold on to its value better when its backed by something more than the restraint of central banks.