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.
Fiat, that is, the national money of each state, will exist as long as the states themselves exist. Fiat may change, experience difficulties, strengthen or fall in purchasing power, but it will exist.
If someone is tired of the dollar, they can always use national money. Nobody forces anyone to use the dollar. But if the majority chooses the dollar, it means it is more competitive compared to other world currencies.