Doesn't matter if there would be millions of cryptocurrencies that has been created when the government decided to take some action. They simply have to ban the use of all unregulated currencies most especially decentralized ones and unregulated ones. When they do that, it will be legal to use centralized cryptocurrency like digital fiat, digital gold, etc.
They don't have to ban bitcoins, and then ban etherium, then ban dash, then ban LTC, then ban Dogecoin. The government is quite smart. They are not going to put a ban on these currencies one by one but rather they will categorize them to ban them all at once.