I think this crypto currency will still not be able to replace the long established ordinary currencies, so in my opinion, it can not replace!
It will not replace fiat since these digital currencies especially bitcoin is not that stable and also, it is volatile, how will the government handle a currency that might take down the economy of their country? I think this is the reason why some of the country elected leaders are banning bitcoin in their country to protect their economy as well as their people in the threat of bitcoin and other digital currencies.