If you ask me if it's realistic the whole world will unite and ban bitcoins, I will answer a big NO. There is no way at this point that countries like Russia, China and USA all work together on one unified approach against bitcoins. The world is too divided to make something big like that happen. In a unrealistic world where everyone gets along and there are no wars between countries it could happen. Then all countries together could issue a ban on bitcoins and prevent stores or companies to accept and deal in bitcoins. This ban would still not mean that a 100% of the bitcoin transactions stop. I am sure a blackmarket for bitcoins would start to exist where the government has no influence.