I don't think that governments will ever prohibit new cryptocurrencies from being developed. I don't see cryptocurrencies as a threat for fiat currencies. They have completely different systems and their goals are also different for people. Rather than these cryptocurrencies, the biggest threat for fiat currencies will be the digital currencies which are developed by governments. Because in the future, they will start replacing fiat currencies one by one.
These kind of thoughts just keep coming that government will ban or do that but it is not going to happen in reality. Yes there are no threat to fiat. Both are completely different things and cannot be compared.