Am I happy guns are off the streets? Hell yes. Would I be happier with sane drug laws so as to not necessitate a black market? Double hell yes.
In that regard: keeping a black market for drugs may even be a good thing. Otherwise they'd move on to other ventures that might be more harmful, kidnapping, murder for hire etc.
Furthermore, you're suggesting that when their way of making money through crime disappears they will migrate towards worse crime. That's a bold claim.