In a situation where the whole world would ban bitcoin and the altcoins. Bitcoin would then surely fall, and everyone would lose their money.

It maybe will last a few years when the ban is new.

I think that the only situation bitcoin will die is by the limitation of available devices and gadgets to have some transaction using Bitcoin. Also, since devices are highly depending on power and or electricity to run, so if there are any interference in a local are like power outage and other natural disasters then the use of Bitcoins will be very much limited. That’s why some conventional cash that are made from paper and precious metals will still be used even if bitcoin became legalized.

If that would ever happen the eventually the economy of every country will also going to have a drop down, or worse currency would be not so relevant anymore. If you would think about it internet is the heart of communication is it not? So If it really happen all the transaction of other countries is not gonna as relevant as it was, although we still have radio communication still internet is the one thing that made it even more stronger and easier.

crypto ban would be huge publicity for crypto.. it will grow even bigger

I think Bitcoin will dead if every country decides to ban Bitcoin permanently. Then, there will be no more people using Bitcoin because they might be arrested in this condition. And bitcoin can also be dead if there are a new cryptocurrency which is more profitable, more advance and can fix most of the bug of bitcoin. In the future, there will be more cryptos qualifying these facors.

we see now that Chinese rumors banned Bitcoin make the price of Bitcoin down about 45%. I can not imagine if the whole country forbids Bitcoin, really you say it is a nightmare.

Anything happening so quick will attract the eyes of government. If bitcoin price has increased gradually, government would have not given a shit about it. Since the price of bitcoin increased drastically, it caught into the eyes of the government. I would not blame the government. Rather i would blame the people. Whenever some opportunity arises, people just go and fall for it immediately.  When people noticed the bitcoin growth, many people purchased huge volumes in a hope  the price will go even further up. Because of huge volumes of buy, the price increased very quickly. Because the price went above the gold, it caught into the eyes of government.  Every government wants their fiat money to be preferred by the people, because government has control over it. So government will not be interested on these cryptocurrencies. 

Anytime government can restrict cryptocurrencies in their respective countries. When the major countries restrict, it is going to be a crash for sure.

But as of now we have relief because japan is supporting bitcoin which does a huge volume. Hope other major countries too support the bitcoins.

may be

the most important thing is that no one else uses crytocurrency, I think that's where it ends.