I think so far history has shown us that a legal intervention in a blockchain has not been possible, and anyway I feel that due to the current adoption of cryptocurrencies in the world, for a government to try to commit this kind of actions would not be well seen by a large majority.
Anyway, time will tell. For now, we can only prepare in case it should happen.
I note that well laid plans are made by patient actors.
We will indeed see.