You don’t fight an evil by adopting it.
Imagine if it was murder we were talking about, and “legalizing” murder was statistically shown to reduce the number of murders. Would you support legalizing it, then?
Now you might say: “You can’t compare drugs and murder! It’s nowhere the same order of gravity!” and you would be partially correct.
It’s not the same order of gravity. Because there are “only” 400,000 murders per year, which is significantly less than the 11,000,000 drug related deaths per year.
Unless you’re willing to do everything it takes to track and dismantle black markets.