Crypto currencies are mostly decentralized, if this will be replacing fiat in the future, then I can conclude that there will be a lot of crypto currency that is not decentralized and I think if that happens a little population of people will just be using that because they don't want to use a crypto currency under the hands of the government.
Most of the people that use crypto currency is because they wanted to use a currency freely and also without that big fees to send. For now these crypto currency will not take over, but if the government decides to use crypto currency, it is obvious that they will make their own, but expect it that it will not be a decentralized currency.