No, they will not go bankrupt just like that.
Banning something takes a while and they could somehow make some arrangements before it happens.
Plus, it wont happen worldwide. They can just go to some country and make a withdrawal there or maybe do their job there.
Governments will have their own decision whether to ban it or not. He could go to Japan if ever that happened in his country.
To the other guy, "disappear automatically" how exactly?
I agree with you. If I may add, you can just do everything online. If the exchange is based on Japan where bitcoin is legal, you will not be put in jail because it's legal where the office of the exchange is located. It's as if you did the transaction in Japan so you don't have to fly there just to exchange your bitcoins into fiat. If that's not the case by the time bitcoin gets banned, then you can simply hire a Japanese guy or contact someone you know that lives there to do the cash out for you.
Banning bitcoins completely will be hard because there will always be a country that will allow bitcoins. It's even a good move because the businessmen that can't do bitcoin business in their country will have to base their business in the country where bitcoin is legal. Thus, that country will collect more taxes which is very vital for a government.
I think those countries that collects more tax to people specially from businesses will allow that banning of bitcoin. But to think on it's advantages to the country, there's no positive effect in either ways progress of the country depends on how the economy circulates from people who spends their money. How can people will earn good amount of money if their freedom from bitcoins will be terminated and those growing business will be shutdown because of that possibilities; negative effects will be possible to happen to the people who used bitcoins for their living.