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Prohibition. Most drinkers were hobbyholics, not hardened gangsters. Which is why everyone stopped drinking and the price dropped.
Only reason people pay lip service to the law is because we are threatened with jail if we don't. It is very easy to use bitcoin anonymously for those so inclined, and there will always be somewhere in the world where using them is fully legal for those not adverse to relocating. Imagine, living like a king somewhere in SE asia because people in other countries are making you rich by breaking the law. However things go, the next years will be fascinating.
Apples != oranges. If bitcoin is booze, so is fiat. Only fiat is legal booze. Your analogy would be accurate if the prohibition banned gin, while everything else -- vodka, whiskey, saki, etc., remained legal. If during such prohibition a thriving gin industry emerged, you'd have a point...
Actually, black markets exist for US dollars in countries that have banned them. So yeah, we are both talking apples. Or oranges if you prefer, Oranges are cool people with me.