Countries and currencies cease to exist regularly. Here's an incomplete list of countries that have disappeared in recent history:

https://www.thoughtco.com/missing-countries-1435425

NO currency has survived a millennia.

Almost all currencies backed only by their governments, and not by physical gold, are at most a century old.

Not true. The pound sterling has been in continuous use since around 800 CE.

That's a ship-of-Theseus fallacy.

The pound sterling of 2019 is more related to the American dollar of 2019 than it is to the pound sterling of even WWII, nonetheless 800CE.

Not really. It’s just a matter of changing times and prices. Pounds and pennies have been a thing for 1200 years. What they buy has changed dramatically.

And additionally the American dollar is not the same as it was pre bretton woods, or pre federal reserve, etc.

The ship of Theseus isn't a fallacy, it's a thought experiment.

And lost 99% of its value!