It's funny- a lot of people are hesistant to buy into bitcoin because they think it might be subject to regulatory attacks by the government. If the government did start shutting down exchanges on a massive scale and making bitcoin transactions illegal, it would actually artifically inflate the value of bitcoin, in the same way that illegalizing cocaine artificially inflates the price by 500 to 1000%. The value of bitcoin cannot be destroyed, and if it's more difficult to obtain them, it will just make them that much more valuable.
Do you really think a business would want to be involved in bitcoin transactions if bitcoins would become illegal ?
I think not, so an average person will not be using bitcoins, except maybe for some illegal stuff.
So declaring bitcoins illegal will end bitcoins as we know it today.
There is a kind of precedent to this situation namely the outlawing of gold by the FDR administration in the 1930s. This did not affect the price of gold in US$ negatively at all, in fact, it rose somewhat.