The issue is that the cartels are now endemic to the region. They're part of the government, they're extorting legal businesses, they're laundering the money and investing into legitimate businesses. You can't just undo their influence.
How does ending the drug war take away 90% of their revenue. If you legalize drugs, the cartels would become legitimate producers and corner the market through fear and intimidation.
#2 Most drugs can just as easily be produced domestically by pharmaceutical companies.
#3 Of the drugs that can't be produced domestically, the Mexican cartels are just middlemen that can be cut out entirely. There's no reason drugs from Colombia need to transit Mexico.
#4 If you're worried about Colombian cartels - see points #1 and #2.
We already went through this with alcohol and organized crime.