This represents a serious misunderstanding of fiat currencies. Fiat currencies are fully backed when issued. When you take out a loan, the bank creates money, hands it to you, and creates a negative entry on their balance sheet. Once you repay your loan, that money evaporates. However, that money when in circulation is backed by the demand for repayment of the loan itself.

A fiat currency will only collapse if you lose faith in the state, your fellow humans and the ability of the judicial system to enforce repayment.

Once that happens you have much bigger problems. Like Mad Max style problems.