This could not happen, not every country is modernized enough to replace their currency for a virtual currency, this would not happen anytime soon, I do not think it will even ever happen.
But this virtual currency has the potential to turn users who are using their bank cards into using their bitcoin wallets. Yes the ration of bitcoin users will beat the traditional fiat users in near future for sure. The raising user bases and bitcoin adopting new merchants are some visible proof to that.