Go to the QuittingKratom subreddit and see for yourself how a legal opioid-like substance destroys lives.
It’s a balance sheet and you can’t just look at the debits and not the credits.
Prohibition does not stop the sale, use and abuse of substances. This is indisputable. The question then is whether or not legalization actually reduces the harm caused by these substances. I believe it does. Both by weakening the black market, ensuring people actually get what they think they're buying rather than whatever the dealer happens to give them, controlling who can purchase things, providing opportunities to educate people before they are able to purchase etc.
All these things are already widely and easily available. Legalization doesn't really change anything in that regard. And most illegal drugs aren't as bad as people are led to believe either. Most of them can be used responsibly with hardly any negative effects, certainly no more than alcohol. The worst part about them is that we don't know what we're getting. We don't know the concentration and we don't even know whether it is what we think it is at all.
There's also the tax revenue we would collect from the legal sale of drugs which could be used to provide education and help those who are struggling.
There is a reason why heroine used to be sold as cough syrup and over time became illegal. I know we want to say the war on drugs was all a war on minorities (and with pot it was), but have you looked into the history of people turning into junkies?