Are you serious? An all out attack on bitcoin would be a stupid and pointless thing for them to do. Consider this. Bitcoin allows some privacy but not too much. That is probably the best scenario for law enforcement purposes in the USA.
The point is that it's possible to catch people like DPR, but still private enough for most people to use most of the time. A "hard" crypto currency that used full link encryption and message mixes as part of the basic protocol would probably replace it and that would be a full on disaster from their point of view.
Bitcoin is law friendly enough to regulate and tax and they'll only lose that quality if they press an attack.
This is an intriguing thought.
Now, if "they" only think that far in advance.. Fair trade!
Ente