fiat currencies will not be replaced totally by the crypto currencies. even though crypto currencies is too risky, with the time it will be also improved to reduce the risk. we cannot say this is not practicable since in Japan there are places that can pay with BTC. it will take time to compete against fiat currencies. but my opinion is that the BTC will be the main currency in future trading(like usd is the dominance in current world tradings). correct me if i am wrong