Wow, that's great I'm looking forward that Bitcoin is become one of the most influential currency in the world. As a global currency currency, I think it will happen in the future and digital currency were accepted by each nation as primary currency in the world.
It could be a global currency but it doesn't mean it will become the main currency of a certain country, it's simply impossible to happen because the government will not allow it. Think of the progress but in a realistic approach, I will be very happy if this will be considered as a payment system of the majority of people which I think will cause massive adoption.
Yes bitcoin will become a global currency but not a national currency because bitcoin is not designed for that and people that will use bitcoin as a primary currency will experience a lot of loss because of price fluctuation so it is not the best idea to adopt bitcoin as a primary currency. Bitcoin will just be added as a new payment system globally, IMO.