The demand for illegal weaponry is not as great as the demand for controlled substances. Yes, there is no doubt criminal organizations would move to other pursuits, but they would be less profitable pursuits.
Also, lets acknowledge the demand for illegal weaponry is already being met by criminal organizations. If you dropped the regulations on controlled substances, some of the existing organizations would try and become legit (we saw this during prohibition). Others would be forced into an already crowded market for dealing controlled arms.