I have no idea.
It could be the keeping marijuana illegal may result in less marijuana users which may result in less gateway heroin users.
Wait, so according to you alcohol being legal has kept many alcohol users from being heroin addicts, most likely because they have a legal means of accquring their preferred product, but making pot legal would increase the number of pot users, and therefore would increase the possibility of the number of heroin users because.....why is this?
True.
It could be the keeping marijuana illegal may result in less marijuana users which may result in less gateway heroin users.
But if you think that the legality of marijuana has a bearing on the gateway effect, you'll have to admit that lowering the percentage of marijuana users that go on to try heroin would be more effective than lowering the number of marijuana users.
A presumed added benefit would be a change in law enforcement focus from marijuana to heroin or other "hard" drugs.