Unless ALL substances capable of being abused are completely available to everybody, a black market niche will exist, which will be filled by the cartels, or somebody else.
Probably anybody can grow their own MJ, but most substances have a cost. People who become completely dependent will have to find a way to pay for their legal substances, so drug related crime will not go away.
For legalization to eradicate drug crime, any drug would have to be free, to anyone (including children) and any of those incapacitated by all those free drugs would have to be provided for by society, lest they commit crimes for food, shelter, etc. No matter how we address the drug problem, society, as a whole, will pay.
DG
Alcohol is a good model. You can buy it at the store, some people make it and kids and a few others try to buy it on the street. How many gangs have gotten rich on alcohol since the end of prohibition?