"When you legalize drugs, they aren't going to suddenly stop selling it. They will now have a legal business which will continue to fund their criminal empires. We most likely will see an increase in other crimes, like extortion and kidnapping."

No. Once a drug is legalized, supply can go up and price plummets. What was once a "criminal empire" now has the cash flow of a 7-11. And because price plummets, addicts no longer need to crawl in your bedroom window to steal money/goods to buy drugs. Crime drops. Drug-related extortion and kidnapping would virtually disappear.