My policy is we need competition and comparison should be in life to motivate ourselves to fix our target and try to achieve it then only your journey of life interesting otherwise you will be very boring. So we should have fiat to fight and perform better then it. In my point of view no need to replace Fiat by digital currencies.

It is not possible to replace fiat because each and every country has their own currencies and its own value so it wont be that much easy to replace yes the volume of usage will be for sure reduced by bitcoin and other cryptocurrencies this is also a fact we have to accept in near future more users involved in using cryptocurrencies and Fiat users will be reduced.