I don't think all countries will use bitcoin as a currency, not because bitcoin is bad or anything, it's just that not all countries are ready for adopting it yet. There are so many countries that lack of internet services in some areas, and of course internet is so important if we want to use bitcoin as a currency. Although there are so many great advantages that bitcoin has, but I think it will be hard. I guess it might be great though, like we all have only one currency, so we shouldn't be confused, or stressed over rates, and other things if we only have one currency. However the implementation won't be that easy.