In the late 80s, the regional director of the Chelybainsk Oblast was executed by firing squad for economic sabotage - possession of US dollars. (Mind you, two years later, nobody cared one whit about his crime, given that everyone came into the possession of US dollars.)

I think it's safe to say that governments will not allow things that will seriously threaten their fiscal policy. For all the wonders of cryptocurrencies, they have to intersect with the real world in order to be useful.