I do not think that bitcoin can ever become a single global currency in the world. He is too volatile for this by nature. Constant sharp ups and downs in the price will not allow bitcoin to become the only world currency, it simply unbalances the economy of any country. Bitcoin can become one of the world's currencies, but not the only one. Yes, I do not think that many states will want to switch to the use of only digital currency, yet the digital currency carries significant risks.