Eh... helping people with mental illness is like 1% of what nicotine does. My aunt had schizophrenia and smoked. It had no visible positive effects and almost certainly shaved years off her lifespan. Maybe she took it for some unknown positive effect on her brain but that's basically a non-sequitor. Doesn't change any of the things people hate about tobacco or cigarette companies. And it certainly doesn't make cigarettes the product in a capitalistic society a net benefit for the mentally ill, even if nicotine the substance might might be.
Also, lol at that article's assertion that smokers have a lower occurrence of Parkinson's. You know what increases prevalence of Parkinson's? Living to grow old.
You refer to addiction and demand as exclusive, but... I dunno. I don't think the concept of the consumer having agency applies to addictive products because the consumer is only completely in control of their first purchase. And for that purchase, all they have to go is secondhand information, which companies like the cigarette companies or casinos can and do manipulate. So the concept that, say, alcohol is "in demand" and thus fine seems a bit weak to me. Sure its in demand, but its nature is to create its own demand and the people selling it know that. This applies to things MMOs too, its the most common trick ever to have the first X levels be super easy to obtain and then start spreading out the dopamine injections.
Yes, people will find some other stupid thing to become addicted to in the absence of specifically addictive things. But that's throwing away "better" because its not "perfect". History* has shown that as availability of drugs increases, addiction increases alongside it pretty much infinitely. I guess you could say "oh they'll find something else to be addicted to" but it seems to me a defeatist argument. The current model for most mental illness is genetic cause, environmental activation. If we create an environment where addictive things are scarce we'll have less addicts.
*specifically the multiple occasions where a government intentionally flooded an area with addictive drugs