I’m with you, and I think this is an important part of the discussion so I hope people will make space for it. Pot, alcohol, even mushrooms can be dangerous psychologically. Society must address this.
However I don’t think this has any bearing on legalization. Whether something should be illegal is a different question than whether it is harmful.
If prohibition, in practice, will cause intolerable side effects, then it’s not a just policy. Even if we agree that the prohibited behavior is destructive.
It’s also not universally true that prohibition even decreases the prohibited behavior. Especially with teenagers it can sometimes have the opposite effect.
I am against abortion bans, not because I think abortion is harmless, but because I think prohibition has terrible effects.
Lastly, there are positive effects of a general culture of freedom. Freedom to make mistakes, not just freedom within a set of formally approved activities. We are able as a group to more effectively explore the space of safe activities if we allow personal exploration of unsafe activities to a degree.
This is why some people advocate not for prohibition of all harmful activities, but prohibition only of those that harm non-consensual participants. It’s illegal to hit your domestic partner. It’s legal to hit your sparring partner at the gym. That’s not because hitting is harmless (it’s not). It’s because many of us think the law is best suited to dampen harmful non-consensual activities, while other means (ministry, fellowship, etc) are more effective means to dampen consensual harmful activity.