Cryptocurrency is just the next layer in fiat currencies, like Federal Reserve Notes which are backed by a nothing but a hot air promise.
A true currency is backed by precious metals.
I use to think like that but precious metal fluctuate and are not stable. What if you have a gold strike like in California? A sudden influx of gold in the market causes problems. Then there is always governments trying to set the price which never works.
Local currency is best measured with a country’s overall economic value (GDP?) and political stability.