I don't think this happens. Fiat currencies are controlled by central banks and governments and do not let this to happen. It will happen only if cryptocurrencies completely replace fiat money. The probability of this is very low.
I couldn't agree more. Bitcoin being use by a massive number of people will not reduce the value of the fiat currency for a reason that bitcoin does not replace it completely and for that the value of fiat currency stay the same. Also as what you have said, governments will not allow it to happen because fiat currency is still what they want and whay they control.
- I really don't see bitcoin being used by a large number of people if we compare the number of users to the world's population, the percentage is extremely low, and with such data, using bitcoin will not reduce the value or affect the native currency. In addition, most users only use bitcoin for investing, trading altcoin, they never plan to use bitcoin as a currency, and as you say, the government does not allow this to happen, this is not a legal currency, combining all the information, we can see the impact of using bitcoin on native currency is almost zero