Do you think that such a situation will ever happen? No never However, if we all know that Bitcoin has been banned together in all countries. Then we can use different exchange. Because Bitcoin is illegal, exchangers will not be banned. The reason for saying this is that the exchanger does not just transform Bitcoin. As well as dealing with currency in different countries. Moreover, if all of Bitcoin use it will be difficult. and At present, the countries where Bitcoin is legalized. The countries will be economically damaged. And the price of Bitcoin will be greatly reduced.  However, we will never see these events.