This was my reply:
An interesting premise. If Gangbanger A doesn't shoot Gangbanger B over illegal drug traffic, then there is one fewer "gun death". But how would that work in the real world??
Congress legalizes meth, let's say. R J Reynolds opens a Meth-R-Us shop on 4th and Main. The government receives tax revenues from sales and all is hunky spunky. No need to shoot anybody over the meth trade!
Except, what about Gangbanger A and Gangbanger B? Are these guys who are used to a fast buck on the street going to put on a tie and use their experience to clerk at Meth-R-Us? Become greeters at Wal-Mart? Find a job where they have to punch a clock or actually work for a living? Or are they going to find some other illegal activity: a drug we didn't legalize, or legalize in sufficient quantities to satisfy the needs of junkies, or prostitution or gambling or cock fighting or human trafficking. Sooner or later, might a territory dispute over the new illegal activity cause Gangbanger A to shoot Gangbanger B?
Or if Gangbanger A doesn't shoot Gangbanger B, but there's no more illegal drug trade to earn a fast buck, maybe Gangbanger B goes into the armed robbery business. Suppose there are casualties among his victims? Hey! At least Gangbanger A doesn't shoot Gangbanger B!