It will be two currencies that will complement each other.
Not the currencies but the types of money. Fiat will be used wider but with the crypto you will earn more traditional dollars or Euros. Thus, they will live together.
I think there will be a harmony between Bitcoin (cryptocurrency) and fiat. Cryptocurrency will not be able to completely eradicate the existence of fiat. But the technology will spread obviously so people would have two choices. One to keep a portion of their income in the form of Bitcoin or any coin and to hold the rest in the form of fiat.
This will be good as well because for precautionary purposes, you need instant transaction and Bitcoin does offer instant transaction but not everywhere across the world.