It would be such a mess... several countries around the world have already set themselves up to be "Crypto friendly" (Switzerland, Greece and Turkey come to mind), so there will always be available loopholes for those who decide to use them. Of course, then we could argue that governments would only aim to clamp down on "most" users (whatever that means) and let the smartest ones slip through the cracks.
It seems to me that the time for "absolute NO" has come and gone, and at this point the best governments can realistically hope for is playing the game in a "reasonable" fashion, in a similar way to how self-employment income is approached. Yeah, you owe it... and it's up to you to report it... and if you're driving a Lambo on claimed income of $18K a year, we're gonna audit the frak out of you.
I'm inclined to agree, @denmarkguy. I'm interested in what you said about Switzerland, Greece, and Turkey, though. What do you mean by "Crypto friendly"? I'll have to check out those countries.