Actually I'd argue that widespread adoption by illegal industries could be the best thing that happens to Bitcoin at this time. The black market is HUGE, it has almost 1.5 trillion market value (according to Wikipedia). If only 0.1% of that jumped on the Bitcoin bandwagon it would drive the price and network hashrate through the roof, improving trust in the security of the network and enabling a lot of people and businesses to work for bitcoins full-time on legal industries, thus creating an adoption spiral. Considering the usual response time of government when it comes to regulating stuff on the internet, we would get a pretty good head start of uninterrupted explosive adoption, making it way harder for them to do something about it.