Fiat currencies are faith based. You have faith that the government isn’t going to collapse and that it will be efficient. Of course this never happens and that’s why they all eventually lose their value.
Currencies are backed by the national economy of a country: its factors of production, industries, exports, resources, stability and overall GDP.
People who say fiat money is 'backed by nothing' are being disingenuous. There's some 'faith', but the value of the currency is also backed by empirical evidence and tangible things, not only speculation.
For what it's worth, the Pound Sterling in paper money form has existed since 1694 and has survived countless wars, disasters, stock market crashes, currency crises, loss of reserve currency status, political and economic problems and has still not collapsed.