Michael Jackson Dead? And legal drugs contributed to his death so... If deaths are this rampant among users of prescribed drugs, that only strengthens the case that illegal drugs, if legalized, are dangerous!
You assume that drug use will increase with legalisation. But drugs are in such ready supply that anyone who wants some can quite easily get some. Nothing sums up the thrill of rebellion more than a teenager dragging on a spliff. Widespread use, irrelevant of classifications and reclassifications, demonstrates that punishments do not deter drug use. However the thrill of risk increases drug use. I accept that it is hard to prove the case either way, in the absence of reliable figures. But that is yet another reason why drugs should be legalised - then you can gather proper data, so that research and treatment could focus around reliable figures, not wild stabs in the dark. Not to mention that such research and treatment would be hugely more successful with a massive budget behind it, funded by newly-legalised drug tax revenue.
But that's all mainly an aside. My real problem with your argument is that you could use that same dodgy logic to say that more people die from illegal drugs than legal drugs, therefore we should legalise drugs to stop people dying from them. It's a pointless and irrelevant argument.