As much as I'd like to leave it at that one snarky word and leave you to fill in the rest, I had better fill out my argument by pointing out that this experiment was actually, factually tried, and it went terribly. Illegal alcohol was far worse for society than legal alcohol. Prohibition resulted in the only flat-out repeal of an amendment of the Constitution ever.
What I was trying to get at was not "wouldn't alcohol cause fewer problems if it was illegal?" but rather "won't legalization of a niche* drug be guaranteed to massively amplify the social ills it causes?".
I have no experience with cannabis, so I'm asking because I'm interested: how do you know that the massive increase in participation which would follow legalization will not cause cannabis to become more of a problem drug than alcohol?
* let's face it, cannabis use is niche compared to alcohol and tobacco
Are you sure that it's not that they're more harmful precisely because they're legal?