For the big country like Japan and USA, i think paper money will dead and digital money will take it. But for developed country, i think paper money will be used permanently.
I think you mean undeveloped countries which can't afford to set up the digital money system. For all other countries which can afford setting it up, digital money will be their only option.
Setting up digital money in underdeveloped countries is not all that difficult. You just need an internet infrastructure and the rest of bitcoin setup is already available online
In fact, you don't even need the Internet infrastructure to make electronic payments possible (i.e. to implement the concept of digital money in practice). The history of such payments had started long before Internet got widespread recognition as a means of facilitating electronic payments (actually long before it got any recognition at all)...
For example, the SWIFT network was founded in 1973 with the purpose of exchanging and processing data about financial transactions across the world