Let me preface this by saying that I'm generally pro-legalization. Particularly of consumption, which when criminalized, makes things worse for everyone.
That being said, heroin is one of those things that are genuinely dangerous to try. It's so easy to become addicted to the stuff, and the costs to society are so high to get an addict clean, that one has to at least consider the pros and cons of prohibition. In an ideal world, all consenting adults should have the free choice to ruin their life if they wish, and perhaps in a post-scarcity society this is what we should allow everyone to do. But while resources are still limited, heroin addicts (and by extension opiates) create a lot of negative externalities. Personal freedom is all good, but where does it end? Should a person be free to ruin the lives of others when they cannot get their fix other than to rob people? And when someone is getting withdrawal symptoms, they have no more free will, they'll do anything to avoid that suffering.
It's tricky to say what would be the marginal increase in heroin users if it was easily available. I agree with you that rational people with well balanced lives and a strong safety net in terms of family and finances are unlikely to go out and buy heroin. People who are bored, in a bad spot, depressed, etc... might just go out and do it if all it takes is a short walk to the nearest shop.
