IF you replace "mistake" with "choice" I would agree with you completely.
Something what might be terrible for you might be wonderful for someone else. I would actually go so far as to say that we decriminalize all drugs.
I say that as a non-user of "drugs", as in, heroin/cocaine/lsd, the kind of stuff they taught us about in DARE. If you or someone else wants to become a cokehead or a heroin addict, then you should have a method and a path to do so that allows for the safest way to do that with the least amount of life-damaging consequence possible for you.
Would legalizing heroin turn the world into an opium den full of depravity? No.
It used to be legal in America and was even sold as a cough syrup that also came with a healthy dose of marijuana and cocaine, and America did not suddenly become or stop being anything other than what it was.
Some things are going to be done by people no matter what barriers are put into place to stop them. Murderers murder. Alcoholics get DUIs, and even with cigarettes costing $10 a pack poor people still smoke them.
If you accept that as a universal rule you can then pivot to find methods to take as much harm out of the inevitability of the event as possible, not only for the person doing the event but also for the people caught in their wake.
For instance, the illegalization of marijuana has caused hundreds of thousands of children to grow up whose fathers are in prison for the non-violent offense of getting high, and has caused even more poor children to grow up in households where their funds were taxed by the harrowing expenses of probation and having a parent who would be automatically excluded from the majority of good paying jobs for the crime of consuming a plant that the second worst president in human history, Richard Nixon, didn't approve of.
Those are social costs that we all have to pay every single day, that is the world our grandparents left for us. Legalizing marijuana and other drugs would put an abrupt end to that social tax, and even though the consequences will continue to be felt for generations, if we stop re-filling the bucket then eventually it will empty, and wouldn't that be a sight worth working towards seeing?