Even if the world did, how could the stop it?
Bitcoin is just currency. No government can stop two people from using something as currency for exchang of good and services.
They may not be able to enforce such a ban in practice, but they could certainly prohibit it by law. I recall that in 2014 the prosecutor general's office in Russia said that using Bitcoin was a crime since it was a "money substitute." By now, Russia has seen a major pivot, and their finance minister recently said that there was no use in banning cryptocurrencies. If it were prohibited by law, I'm sure that would put a damper on investment, but indeed, it wouldn't fundamentally change the utility of BTC.
Let's get some examples of how a government could ban Bitcoin. China hasn't even banned Bitcoin, they just won't let you buy Bitcoin in China with yuan.
See the OP. There was a leaked report recently that suggested that China might actually attempt to block the Bitcoin network within China, similar to the way they block many western websites today.