Fiat will eventually fail. The problem is they're trying to use inflation to "inflate away" the debt, but actual markets can only deal with that for so long. They have the money printer; we don't.
Inflation is currently a global issue, though it is worse off in underdeveloped countries and regions that have an unstable economy, too many governments around the world have been printing money out of thin air since the period of Covid-19 in order to solve short term problems and create long term inflation.
Notwithstanding all of these, fiat isn't going to fail, at least not the fiat currencies of countries with a stable and healthy economy, as for the countries that do not have a viable economy and with a currency that is as good as worthless, they will probably end up adopting a stronger fiat currency, like the U.S. dollar for example, as many countries do already.