I meant to ask what would prevent today's illegal seller from becoming tomorrow's legal seller.
DGP, I have wrote a ton of my economical answers for that question:
Lets say I have a large parcel of land that has been proven to yield wonderful apple trees. The apples that come from my land are the best of the best and can only be grown in my general area. Now lets say you live in a place that is too cold year round to grow apple trees, much less the great apples that I grow on my plot. If you wanted to purchase my apples you'd have to pay a lucrative markup as I would have to get them to you. You love my apples enough to pay the extra for shipping as well as technology/energy to keep them fresh. Our business arrangement is contingent on the fact that you cannot obtain apples of similar quality without getting them from me.
Then, suddenly, someone discovers a way to grow equivalent apples in an easy way and not requiring the perfect ground soil and weather. My business model has to then be refactored. The price I can set my apples at has been dramatically reduced because my past customers can now obtain what once was a rare product with ease.
What drug cartels are going to have to do is start selling their weed to people within their country alone, where it still remains rare because of government prohibition and therefore can be marked up. Their market will be neutralized and no amount of extortion will make their weed products any less commonplace after the US prohibition is lifted.
If the cartels really want to save their little niched black market they should infiltrate US congress, while they have to capital, and keep weed from becoming legal. Because once it is it will fall off their shelves along with all the capital that it gives. They'll have to start buying bathtubs for meth production to make up for the loss of profit from the legalization of weed. It would be nothing short of devistating, DGP, you WANT the US to legalize weed, believe me.
^ Not all drug dealers are businessmen. So with the legalization of a black market product, the drug dealers will be forced to become businessmen. The ones that have no idea how to create a good product will crash and burn, and like you say, turn to their normal extortion, pimping and dealing in OTHER illegal drugs which is pretty much what we have now except weed is on their shelves. It's too alluring to take it from them and place it in the real market's shelves. It will be a mortal blow, I hope.
-Sab