Weed is a gateway drug in more ways than one. Many criminal organizations that are into trafficking hard-core dope, got started cultivating or dealing weed. The weed provided them the capital to expand into coke, meth, and junk. This travesty of a law will provide the Mexican drug cartels will even more customers in the United States and will produce even more brutal murders on both sides of the border.
That's true, but probably more important than the capital is all the connections they'll have. They have customers who like to party, some who are into other drugs too and others who might like to try a little of something else, and they have the people they get their pot from who can probably get other drugs to through the same channels where they get their pot. And often it doesn't take any capital. Drug dealers selling to people who sell drugs will often “front” the stuff to regular reliable customers they trust, give it to them on credit. They'll say something like, “If you think you can move a quarter ounce of coke I'll front one and you bring me the money next week. I'll sell it to you cheap enough that you can take a couple of grams for yourself, add in a couple of grams of cut to the rest, and you'll still make a little extra money if you get ‘x’ per gram.” It will work something like that, or they'll just start picking up a gram or two here and there for their pot customers and that will grow into a thriving coke business for them. It's all the same black market basically.
If we legalized pot we'd dramatically shrink that black market and the number of participants in it who are all potentially going to help distribute the drugs. It's almost like Mary Kay cosmetics or any of those other multilevel network marketing companies that recruit from their customer base. Remove a huge portion of their customer base and they lose sales and huge portion of those they would have recruited to make more sales. Take marijuana out of the black market and make it a legal product that is produced like other legal products and goes through legal distribution chains and it would rob the large drug trafficking organizations of most of their revenue and an awful lot of the people they would recruit to sell the hard stuff, and it takes a lot of the end consumers out of the loop for the hard stuff too because licensed “pot shops” aren't going to be any more likely than liquor stores to sell drugs like meth or cocaine or heroin. Young people, “party people,” will be less likely to have the opportunity to use these drugs. I bet people take on a much less charitable view toward illegal drugs and illegal drug users after that because most of them won't just be people who occasionally smoke a little weed. They'll mostly be hard drug users. The number of illegal drug users would instantly shrink dramatically simply because the illegal drug that is used more than all other illegal drugs combined would no longer be illegal. The few “illegal drug users” that remain would be using drugs that most people, including most pot smokers, take a dim view of to begin with.
There is no doubt in my mind that legalizing marijuana would be devastating for the huge multi-drug trafficking organizations which are mostly run by Mexicans now, and I think it is quite possible that it would lead to a fairly substantial reduction in the number of people who use the hard stuff.