i think this one will not happen and if it is someone or somewhere will do an action to exchange bitcoin to fiat.. if all the countries will do this then no more bitcoin
I never think that scenario will happen, but even if it will happen, it is useless, people are looking and creating any ways or systems that caters cryptomarket transactions. On the other hand, they will not ban exchanges because many people are benefited and changed their financial stability through crypto business industry.