I think it's good. I think making it forbidden makes it more desirable. I think the privatized penal system thrives on the war on drugs, which created more crime, more "need" for police and courts and systems of social control and abuse. Prohibition didn't stop alcohol consumption, it made it more desirable and more dangerous to access and consume, and more seedy businesses popped up in response, just as seedy privatized prisons popped up in response to the war on drugs, as well as all the nasty businesses that pop up to make and distribute the drugs. Gangs don't have any business if things like drugs and prostitution are legal.
I get that alcohol and drug use cause problems including domestic violence, but banning such things doesn't make them go away nor solve the problems they cause. Likewise, I don't care for prostitution/sex work because of deleterious impacts on the worker, but that doesn't make the profession go away. I admit I've never been to Amsterdam, so I only know what I've read, but sex workers get benefits like other "legitimate" workers, they're not (nearly as) stigmatized, they get health benefits to care for and protect their bodies, and because they're not (as) stigmatized, they can report a violent perpetrator with more confidence and be less judged, less likely to be told, "You asked for it." In the same (haha) vein, how many people OD or are crime victims related to illicit drug use and don't get help because they or those who would help them are afraid of getting arrested?
While the use of substances and alcohol causes serious social problems, the drive to partake in them doesn't go away. I don't know to what extent this is true but it's worth considering and contemplating -- I read once that every animal has a way to get inebriated or high. I specifically remember reading about elephants and what plant they would seek out, ingest and zone out on. I think there may be a biological function to seeking and experiencing inebriation, and some folks can manage it better than others. Indigenous cultures have been using plant medicines since forever, and there are some people who can use them with purpose and have positive experiences, and some that just get fucked up by them and seek to abuse them, who probably are intentionally locked out of the shaman's peyote or ayahuasca supply. It's my underwear that Native Americans don't have the biologu to manage alcohol, and when it was introduced by Westerners, it has massively deleterious effects on their culture, but at this point, prohibition wouldn't change that, so I concede you have a point about the slippery slope; there's no win, maybe only mitigation, maybe things will be worse.
Substance abuse will always be a problem, and substances cannot be eradicated, any more than prostitution can be eradicated. Better that women be their own bosses instead of have abusive pimps, and get benefits. Better that substance users have legal access to substances and legal possession and medical and protective services, rather than have to go through abusers to get the substances, and have no safe services to support them if things get out of hand.
Better that abortion was decriminalized and legalized, it saved bodies and lives, because abortion isn't going to go away either. Better that humane euthanasia be decriminalized and legalized as well, because suicide isn't going to go away either, and failed suicide attempts cause great harm to those who attempt and put burdens on social support systems.
God, life and social systems are so complicated and fucked up. I don't think anything can be "fixed." Sometimes it weighs so heavy on my spirit. It seems like whenever there is liberation of some sort, then there is resulting harm somewhere else.