That won't happen. Bitcoin and many other cryptocurrencies are digital versions of money, but their birth and development were not aimed at creating a cashless society(fiat).
But with the assumption that it is possible, it can be seen that everyone is at home safe, and only needs the internet, purchases and payments in bitcoin. LoL
Even lets say that this do talks on that "what if" situation then i cant really put up into my own brain or even on my dreams that this thing will eventually happen.
There's no way that this could happen.It would really be just remaining as an alternative and to say that digital money had been existing for long now.
Crypto might exist but this would be on different manner or characteristic but wont really be ending up on being a mainstream or would be primarily used.
Government do exist and as long theyre here then it wont really change a thing when it comes to monetary sector.