See:
There Is No Escaping History. Fiat Currency Eventually Fails.
Notice that this is a critique of fiat money -- not a critique of state-issued money under a gold/silver or other standard. Fiat money is a rare occurrence in modern history. (We live in strange times!) As we can see, the examples found by the author are a small number of short periods here and there.
I have my own critique of gold/silver standards, but that's another issue.
Fiat will fail depending on the economy, the price of cryptocurrency will decrease depends on the inflation rate while cryptocurrency is just basing on supply and demand, that's the reason i prefer digital coins rather than fiat.