" I would say nearly all the harm comes from the fact it's not legal"
Wow, you're not using your imagination well enough to recognize how ubiquitously widespread opioid addiction would be were it integrated into our coffee, soft-drinks, an aperatif with dinner etc.. It would be a massive, ongoing public health crisis.
Combined with the fact that the effect quickly erodes, that users want ever more aggressive substitutes like Fentanyl, it almost guarantees the inevitable OD.
Blaming the 'harm' on systems literally trying to stop the promulgation of what arguably is actually poison ... is a leap in logic.
Tobacco is similarly additive, but it obviously has a completely different set of health effects.
Finally - if Tobacco is really in the same category of addictivity, then ironically, you've made an argument that we really should shut down the opioid industry hard, because while quitting smoking is difficult, it's not existentially hard and the resulting withdrawal does not require medical attention or anything on the scale of 'Trainspotting withdrawal'.
An acquaintance in Prison indicated they call the heroin addicts zombies, because they bang on the doors and yell 3 times a day as they receive their poison, which is slowly killing them. I'd argue that incarcerating people and slowly killing them is a strange punishment for those wanting to 'reduce harm' whereupon the logical thing for their health would be to make sure they had absolutely no access and an opportunity to dry out. I hope that in 50 year we look back on 'free heroin' like lobotomies or shock therapy.
Humans have some control of our faculties and must overcome impulses of all kind - facilitating the total degradation of that control system by administering poison is definitely a form of torture. I'd rather be waterboarded.