Actually, after the U.S. ended alcohol prohibition virtually all of the organizations devoted to booze either went legit or drastically shrank in size and scope. We would've been better off without alcohol prohibition in the first place, but ending it was certainly a net win.
If you're interested in the history and parallels of alcohol prohibition to today's drug prohibition, Daniel Okrent's Last Call: The Rise and Fall of Prohibition is pretty good: http://www.amazon.com/Last-Call-Rise-Fall-Prohibition/dp/074... .
It's not impossible that the problem would just poof go away, but it seems a poor choice to bet on it. Again, that doesn't mean that legalization isn't the place to start!