You're already living in it, killer. Caffeine, for example, is a pretty powerful stimulant. Ditto for nicotine.
Alcohol is everywhere. Alcohol consumption correlates heavily, like crazy heavily, with assaults, rape, murder, domestic violence, accidents, and serious or fatal car crashes.
Weed is now legal or legal-ish in many places, essentially all of the big population centers in N. America. Edibles and vapes mean you don't even have to smell it.
And let's not forget all of those articles about SV wunderkunds "micro-dosing".
The addicts that the OP was discussing have serious withdrawal issues and are, compared to things like booze or marijuana, trivial in terms of overall population.
They're already doing it. At least this way they won't have as many horrible skin ulcers. The situation is inhumane.
Consider the principles from another perspective. The leading causes of death in the US are diabetes, heart disease and obesity related conditions. Should we prohibit fast-food franchises, TV dinners and sedentary lifestyles?
As you said, "Do you want to live in an obese society?"
Logically, opposing prohibition isn't the same as endorsing obesity or drug abuse. There is an important distinction here.
It's the world we already live in. Continuation of the war on drugs mentality just leads to horrific outcomes like the one being discussed. And those outcomes have a wildly outsized impact on people who were born into unfortunate circumstances.
It impacts us all as members of society. Adulterated drugs are killing US addicts in record numbers.
There is a direct social cost in terms of health care, law enforcement, crimes committed to obtain drugs, fallout from broken families resulting in further dysfunction and more.
Then there is also the unseen cost of lost productivity. If users, deceased users, imprisoned users, distributors, deceased distributors, imprisoned distributors, and law enforcement personnel were not engaged in these activities, their efforts could be focused on more productive areas of the economy. We all pay for this policy socially and economically.
Same concerns about anti-depressants, which commonly make you unable to cry, have sex and are associated with increase frequency of mass shootings and suicide?
