I don't think it will ever become a default currency for modern nations. I think it will be used for parallel economies, which will expand. People will be able to buy more and more goods and services with crypto. Many more services will be set up to facilitate the conversation from USD, for example, to BTC. The scale of these parallel economies will depend on the lengths to which governments attempt to stifle freedoms. As they become more egregious, more and more people will opt to be paid in cryptocurrencies, and shop at places which accept it. There are many ways governments can and will clamp down on cryptocurrencies, but there are even more ways to achieve circumvention.