If this occurs, it would be in the future if they have replaced bitcoin with a much superior currency. Since bitcoin is decentralized, and there are a slew of resources to use on the internet to gain access to cryptocurrencies, the government can do little, even though they go all out against it.
That superior currency definitely will not remain decentralized; that is the point on-which it may get failed over the time even if governments unite and plan up anything like that. As of now we are seeing some governments are planning for their own version of digital currencies and some other governments are about to bring heavy regulations etc. But all these will fail for the same decentralization reasons. Bitcoin's biggest power is its decentralized nature hence anything against bitcoin without that will fail over the time.
There are very less possibilities for governments to come together to think about stopping bitcoins; even they take any step on this regard that will end up biting dust for sure due to same decentralization and benefits based reasons.