In my opinion, it's more of an adition to the today's currency.
As qouted in this article
https://www.linkedin.com/pulse/enjoy-bitcoin-new-era-cryptocurrencydigital-currency-azad bitcoin is the new era of currency. And I agree to this statement that bitcoin is the key to the new era of currency. Blockchain technology will be copied and will be widely used even by banks and other insitutions. With the success of bitcoin as a good example banks are considering to launch a blokchain based fiat digital currency. And among other countries maybe the first country to go cashless will be China since they will release their fiat digital currency prototype this September 2017 to replace the Chinese Yuan.