Cryptocurrencies especially Bitcoin were not created to replace fiat. They are even meant to be pee-to-peer currencies unlike fiats.  Do not forget that fiats exist in digital form, so they can actually be programmed to do what Bitcoin does.
If the use of bittorent did not stop the use of controlled alternatives why would Bitcoin stop the use of fiats? I am really not in support of the destruction of fiat. They can all exist side by side.