Crypto is always reserved to be used as a substitute for fiat currency but it is difficult to realize because if crypto must replace fiat currency then there must be regulation from all countries in the world related this crypto. In addition, there is a need for an economic balance of the transition period from the exchange of this currency.
But if Cryptocurrency replaces government cash, it will be very costly for this, the cost of preparing machinery and technology to meet the needs of the people's BTC and some knowledge of technology. Therefore, in my opinion BTC can not replace fiat money forever after.