By your reasoning, though, one might surmise that ending the drug prohibition would then also end gangs, wouldn't one? The gang exists to defend illegal drug territory. Illegal drug territory only exists because drugs are illegal. Ending prohibition would make drugs legal, not illegal, and thus the outsized profits would go away. This would then make operating the gang unprofitable and it would disband or "go out of business".
I'm sure you'll bring something else up like "but drugs are bad!" or some such. But that line of reasoning works, doesn't it? Changing the subject because you don't like the moral implications of legal drugs isn't an argument.
Forgive me for accusing you pre-emptively of moving the goalposts. It happens a lot and I get tired of it. That's usually what happens once it becomes clear that legalization would end gang violence, another objection to stopping legalization for another, never before mentioned reason.