If one of these coins are REALLY successful, it will be a threat to government power and they will try to suppress it. It will also threaten the status of USD as the de facto international currency. I just hope they can't take it down or by the time they try, too many of their citizens have a stake in the coin to just let them shoot it down.
By really successful, I mean a lot of people worldwide actually use it to trade goods and services, not just a small fraction of the world population hog it hoping it's value will increase.
You will still need to pay taxes to the government and they will still annoy you about it.
For a government, it is just a matter of thinking how to do this.