Earlier quoted context omitted.
Well, the government could make undeclared cash illegal.
It might even work, if they had a way to track all your cash. Unfortunately, unlike with Bitcoin, they don't. This is pretty much what I said in my original comment? I don't understand all the apologism, it's pretty clear that governments can exert an unprecedented amount of control on money with Bitcoin (all your transactions are public), and no amount of burying our heads in the sand will change that.
I would recommend to familiarize yourself with how bitcoin works on a technical level before mowing down more strawmen.
Also: Do you really think all countries of the world will agree on one central 'bitcoin address registry'? Contrary to popular belief the internet does not end at Virginia beach...