I've found that people don't really care about the legal status of an activity when it's easy to do it without punishment. See copyright infringement as an example.
Then you'll also know that the people care no matter how easy it is far more when the severity of punishment goes up, and if BTC were serious enough to make illegal, then surely (like all serious financial offences) punishment would run high. It's easy to rob a shop, but stealing is a serious crime and barely anybody does it, but many people pirate movies off of the internet and the punishment for being caught is pretty low.
This can all be solved by moving the bitcoin network over to a darknet (such as onion or i2p), in which case the government(American) can only block people using those services (which it currently can't do).
No the real target when the authorities want to clamp down on bitcoin will not be the network itself, but the exchanges. It is the exchanges that give bitcoin a lot of it liquidity, and it's exchange value gives people more confidence in it's usage as a currency, right now there are far too few goods and services that can be bought with bitcoin so the exchanges are crucial to it's healthy growth in use.
Shut down the exchanges and you kill bitcoin, it also doesn't help that the exchanges themselves are very vulnerable to authority action.