Gangs will always be in business even with legal pot. Just look at cigarettes and alcohol. Both those things are legal and both those things have massive black markets to bypass the heavy regulations and taxes imposed on them.
Are you speaking on the US or worldwide? A quick search doesn't produce any reliable statistics that indicate this. I have never once heard of anyone purchasing alcohol on the black market. Cigarettes, sure, but even then it's typically only very low-income folks purchasing cheap, illegal tobacco that is generally not of great quality. This is in stark contrast to marijuana where high-grade product can be purchased on the "street".
I suspect that once(if) pot goes legal the black-market will consist mostly of cheap, poor quality marijuana. This should take a nice chunk out of the profits that gangs see from marijuana distribution.
You can get hooked up with illegal alcohol in basically every liquor store in every state in Appalachia, if you know someone that works there. It's commonly sold under the table so to speak.
edit: for wherever the downvotes are coming from - yes, I do know this stuff for a fact. I grew up in Appalachia, I know for a fact that what I'm saying is extremely common, I saw just about every aspect of the illegal alcohol trade first-hand. From that I came to learn second-hand the scope and nature of it up and down the east coast of the US and other parts of the country. It's easy enough to google this subject as well if you're skeptical.
But large criminal syndicates funded by blackmarket alcohol are very much a thing of the past. At least in the USA.
I guess I technically know quite a few people who make "black market" whiskey. It's not legal to distill without lots of licensing and these people do sell bottles to friends and friends-of-friends.
But it's Really Hard to understate the difference between poor folks running cottage industries around moonshine and the sprawling violent criminal enterprises that flourished around the alcohol trade during prohibition.
(e: also, illegal alcohol isn't just a poor person thing. Plenty of wealthy folks illegally distill, they just normally aren't motivated to sell.)
Certainly. And it's some gargantuan scale different versus the illegal drug trade as a comparison (0.1% as large perhaps). My point was that it isn't particularly uncommon, the underground illegal alcohol trade (it's also not monetarily massive in size as you're noting, which is why it exists at all, local police mostly look the other way so long as it stays small'ish).