That's a good point, but for the case of illegal online activity bitcoin currently has a kind of anonymity guarantee that no other system does, and is as such a

That's a good point, but for the case of illegal online activity bitcoin currently has a kind of anonymity guarantee that no other system does, and is as such an apples to oranges comparison. If the USD had the same anonymity guarantee you'd see those people making the same choice everyone else is: hold onto bitcoin and spend USD.