Cryptocurrencies are volatile and decentralized and i think that  the government will never allow something uncontrollable to get into their economy and never being used as the main currency. There will be a cashless society but it will be not with cryptocurrencies. But if this will happen, the closest and most possible coin that will be use if possible bitcoin is the most preferred.But I don't think this will happen in the future, they might implement some cryptocurrency that is centralized so they can control it, the supply, the price and it's marketplace.