That wouldn't change anything regarding the guns or the amount of crime - career criminals don't do it because they believe in selling drugs, they sell drugs because it's an easy way to make money.
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.
People don't get physically addicted and spend their every last cent on murder for hire after murder for hire. Think of how stupidly lucrative a substance like crack is, it is water to an addict.
"career criminal" is begging the question. You claim these people exist, and by definition they are going to continue crime rather than find legitimate ways of making money.
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.
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.