Mere possession can be outlawed, but those laws can never be successfully enforced.
Example: firearms - there are laws all over the world regulating the possession of firearms, and those laws are utterly ineffective in their attempt of preventing the illegal firearm possession. Those laws are only effective if applied to the legal and dully registered firearm owners - which ironically are the only ones who comply with those laws.
Same with Bitcoin - if the governments outlaw the Bitcoin, the only Bitcoins that could be (easily) confiscated will be the ones deposited with legally compliant institutions - the ones stored in a private wallet could never be effectively confiscated.
Time to post the Bitcoin version of a "MOLON LAVE" poster, I suppose. :-)
The way they will take down Bitcoin is by linking it's users to CHILD PORNOGRAPHY. If you use Bitcoin, Tor, etc, then you're a pedophile with a pedophile kit. Why else would you need all that?
And if accused of being a pedophile it's near impossible to prove you aren't when Nancy Grace or whomever in the media crucifies you.