It's really just a simple waiting game. There is a strong need for a world reserve currency (currently filled by USD, and somewhat my EUR). Fiat currencies have a 100% failure rate
No they don't.
There are many many fiat currencies currently in existence, and those haven't failed.
Your statement boils down to "Fiat currencies that have failed have a 100% failure rate", which is content-free.
On the same basis:
"Democracies have a 100% failure rate"
Still being in existence is not proof that they will always be in existence, especially in light of dozzens that failed throughout history, versus the handful that still exist. Many of those that currently exist are extremely new too (Euro is about 20 years old, USD as a fiat is only about 45 years old). Commodity based currencies, on the other hand, don't fail, and just get supplanted by something better.
I don't know about democracies, since some failed and others have just been conquered and wiped out, but empires historically do have a 100% failure rate.
As for wars, that's not just "country was conquered," but likely includes "currency was used by country to pay for war, leading to high inflation and collapse. For USSR, the crappy economy was one of the reasons their currency failed, them printing and spending it like crazy to build up their war machine is another.