I see your point, and it is relevant for the discussion. Still, I think that's again a rather deterministic view of human nature. We had slavery for many centuries, and we eventually realised it was inadmissible. I'm just pointing out that attitudes may change. For example, there has always been and will always be murder, but we should strive to curb that.
Now the natural reaction is: drug use doesn't even compare to murder or slavery. Of course it doesn't, but I'm just trying to prove by making these analogies that
1) just because there has always been [blank] doesn't mean there will always be;
2) just because there will always be [blank] doesn't mean there should be.
Ultimately, then, all we have to take into account are the drugs themselves. My life philosophy is about life-affirming actions. A relative of mine is a judge and through him I've known just too many lives destroyed by hard drugs, it's very depressing the number of kids and adults who steal, abuse physically and verbally and even kill their parents so as to get money to support the addiction.