I think you are assuming a great deal when it comes to legalization leading to greater consumption of illegal consumption. For the sake of argument, I will say that I agree with you: On the margin, while I may not be a person who is dissuaded from using a drug from it being illegal, there may be someone else out there who is. My point is to say that those handful of individuals who are frothing at the mouth to consume drugs, but are dissuaded by its illegality pales in comparison to the number of lives that could be saved should drugs be legalized - since it seems making drugs illegal doesn't cause as drastic as a decrease in demand as one might think. As for your example of The Purge, I am so glad you brought it up. One of my favorite living economics Bob Murphy has an episode on it pointing out some plot holes that are more obvious to economists. I cannot make you do a special assignment, but if you listen to this podcast episode and respond to it in the next forum post, I would certainly give out some brownie points (however much those are worth).
https://www.bobmurphyshow.com/episodes/ep-154-bob-murphy-analyzes-the-hunt-and-the-purge-movies-as-an-anarchist/
https://www.bobmurphyshow.com/episodes/ep-154-bob-murphy-analyzes-the-hunt-and-the-purge-movies-as-an-anarchist/