"As long as you can exchange bitcoins for drugs delivered to your front door in first-class packages, it will be fairly trivial to exchange bitcoins for cash if you really need to."
Sure, but having it declared illegal would definitely restrict the growth potential of the currency. At the moment its proponents envision/fantasize about it becoming huge and general-purpose, with a presence in the physical world (see bitpay etc). If a large nation like the US were to declare it illegal would certainly put a dent in that.
I disagree with a lot about BTC, in terms of the economic choices made in its model rather than the technical ones, but I find it continually fascinating to watch it as a phenomenon.