Frankly some stuff is really bad and should be kept away from people.
Safe injecting facilities are not about helping people be strung out. They're about reducing harm, both to the addict and to the rest of society, coming from drug addiction.
I consider myself fairly well insulated from drug use. In my entire social group I know of only one person that has ever tried methamphetamine (which is rampant here in Australia), and none that have tried heroin. Despite that, if you set me to the task of aquiring heroin I'd be pretty confident in my ability to do so. I doubt I'd have to go through more than 3 levels of friends to find someone that could hook me up. And that's not even taking into account darknet markets (which I imagine would be fairly easy to learn how to use as a software engineer).
If I woke up tomorrow and decided to try heroin, there wouldn't be very much in the way of obstacles in my path.
So for me at least, there's not that much difference between the current situation and legalisation. Especially if said legalisation requires a prescription. Arguably convincing a doctor that I'm an addict and in need of heroin is more of a barrier to entry than finding someone that sells it now.
1. Buying it illegally funds criminal organizations
2. Quality is terrible, it's certainly adulterated
3. Association with low class and poor people
You can ingest or smoke opium products, too, if the needle is the off-putting part of heroin.
But yeah, another anecdotal perspective: I've had prescription opioid painkillers but don't have any particular interest in continuing to use them after the pain goes away. So if opioids were legal tomorrow, I don't think it would change my behavior at all either.
I think it's maybe too early to tell what impact CO/WA/etc legalization has had on MJ use.
Most of the poorer Asian countries have been like this in my experience.
Heroin isn't too different, really. It's very addictive, has a low LD50, and when it's easy to get, people effectively get a free ticket to a brothel on an accelerating rocket-fuel train with rarely stops. If a drug can have that much power over the body, people might think we could create microdoses, and sell those, and then it'd be safe. But then, the market for heroin/tar/etc. would still be there, and would continue underground.
It's not as simple as "selling it at CVS w/o prescription will lower overdoses and dissolve the underground H market"