Still being in existence is not proof that they will always be in existence, especially in light of hundreds that failed, versus the handful that still exist. Many of those that currently exist are extremely new too...
Is that about currencies or democracies?
Both? Note many of us don't support fiat nor democracies.
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Commodity based currencies, on the other hand, don't fail, and just get supplanted by something better.
Many nations used to have commodity based currencies. Few (None?) do now.So either they failed, or you think the fiat currencies that replaced them were better?
They were better. It is safer and easier to carry a paper note than gold coins or gold dust. The problem wasn't with gold backed notes themselves, but the lack of trust in people who were issuing them. Plus democracies that realized it's worthwhile for them to inflate money to give it away as social programs, in order for the voters to keep voting for them.
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If you wait long enough, everything has a 100% failure rate, that is the point.
Has the nurse and buggy failed? What about the fax machine? Or the musket? Good technologies and ideas survive, and get replaced, but still work. Gold still works. Fail implies "breaks and stops working." If you wait long enough, everything will get replaced by something better, but not everything "fails."
...but you don't seem to apply this logic to fiat currencies. It seems that by definition, any time they are replaced, it is because they have failed.
If a country gains independence and changes its currency, that is counted as a failure of the previous currency. If it is conquered by another country and changes it currency, that is counted as a failure of the previous currency.
If currencies are subsumed into a single currency union, that seems to be counted as a failure of those currencies.
If the above isn't what you think, then you must agree that there are many fiat currencies which are no longer in use, but which didn't fail, by the definition you used for commodity based currencies.