I never said it wouldn’t happen again, in fact in my next post to you I outright say it could. Of course making them illegal doesn’t mean you stop them. Opium was illegal in part or in whole in China for your ENTIRE graph (medical use only as of 1729, banned completely 1799). Which hits the usual problem the WOD defenders never seem to be able to acknowledge: making something illegal doesn’t keep people from using it.
You’re right about one thing, people should study that period, and use it to understand that making something illegal doesn’t prevent use. You’ve got to work on your society to get people not to use drugs. Plenty of places had opium legal during that time frame and didn’t have the massive problem China did, and it was illegal in China. If you’re going to use this time frame as a “what to do” model the time frame says legalize, and nuke England.
It's a numbers game. Using absolutes is silly when the real measure is percentages. Sure, 2% of the population can get drugs because they have contributed much effort to maintain a black market trade in this stuff. The black market couldn't support more than 2% of the population for any length of time and this keeps the stuff from spreading to the other 98% of the population.
You may or may not have experience with drug addicts, but I have. I remember people I knew who would frantically dial every pusher they knew as well as other addicts in a desperate attempt to find someone who was "holding." They would run all over town going to various drug hangouts just trying to score a hit because they were jonesing.
So to answer your point, no, making something illegal doesn't stop everybody from using it, but it stops most of them.