Nothing will happen to them because fiat currency will always be a valid means of exchange. The moment we start looking forward to a time when cryptocurrencies will completely make fiat an outcast, the more we appear to be extremist. In the world we live in, I believe that the so called illiterates make up the majority of the population. Trying to take them out of the equation will mean that we're counting them off from life.  The bottom line is that it will be a matter of choice what each person seeks to use for exchange of goods and services. If one does not understand cryptocurrencies, fiat will always be there.