Look at it like this. Once private companies start selling legal goods that were once illegal, the illegal products for the most part are no longer sold by the illegal dealers. We don't have speakeasies any more because prohibition is no longer a thing. If the government taxed marijuana and made it as available as cigarettes, we wouldn't have near as high an issue with illegal dealers.
In fact, there are 700,000 indictments on Marijuana every year in the U.S. We can take all the money that we save not sending cops after users of a relatively safe recreation drug and combine that with money obtained from the weed tax and reinvest that in our terrible economy. It's a win win win situation.