We already tried this experiment once. Making alcohol illegal did two things.
It actually reduced consumption and alcoholism.
It greatly increased the amount of black market crime, and it drastically changed how it was consumed.
Harm in vice will always occur. We just have to decide if we want it in the form of overdose, or the form of black market versus police state violence.
Heroin addicts usually know how much of their usual dealer's heroin, cut to a particular purity to take to get high without dying. If they take the same dose of a more pure drug, they overdose. If they take what they think is heroin, but is actually fentanyl, they overdose.
Likewise, during alcohol prohibition, liquor containing methanol was a common risk for poisoning when drinking black market alcohol. Even the cheapest grain alcohol at a college liquor store today will not contain a dangerous amount of methanol. Legal heroin would reduce this kind of risk in the same way; it would contain what it says it contains, in the concentration on the label and would not contain other harmful substances.
I think this is one of the key insights. Although, thankfully, I think it's more than a binary choice (either a) legal or b) illegal).
Regulated legality combined with heavy restrictions on marketing and sustained support for rehab / support / education seems to me the best of all worlds. Still keep things accessible enough to cut out the motivation for the black market. But don't let the less scrupulous side of legal society run amok and actively exploit that.