Why wasn’t marijuana even considered banworthy prior to the 20th century. For that matter, prior to the end of alcohol prohibition.
I'm sure you are aware that the legislative history of marijuana prohibition has precious little to do with public safety or helping people be all they can be. ;0)
In the same vein, the other drug problems we had in those years were due mostly to ignorance (and the many wounded Civil War and WWI vets). From what I have read, people weren't taking patent medicines to get high, they were taking them in good faith as medications to fix some malady. We now have a much better understanding of addiction and the effects of various substances on the body.
That's why I am pretty confident that legalizing even harder drugs like cocaine, heroin would not result in a dramatic increase in users.