You don't have to be in favour of the current war on drugs to be in favour of heroin being illegal - and this is key selling heroin being illegal. Even Portugal, held up as the example of decriminalization only decriminalized possession not distribution.
Being a heroin addict should be treated as a social/medical problem and people given all the help they can to get clean, but we absolutely shouldn't be encouraging people to take it up. Especially heroin which is responsible for a very high percentage of all drug related deaths.
I think you've struck upon the fundamental issue when it comes to substance legalization: we've made the U.S. government the arbiter of safety. People think, "If it's legal, it must be safe."
That isn't a true statement even now, but it's the mindset people bring to discussions about decriminalization.
Legalization does not need to mean "encouraging people to take it up." If we want to end the war on drugs, we need to all accept responsibility for determining what is safe for ourselves. The government's role should be in providing information and support.
Don't forget marketing. Alcohol is marketed extensively by the industry despite the associated problems. A legal heroin industry would have a huge incentive to market heroin.
Heroin + other drugs combined are responsible for a high percentage of all drug related deaths. You can make the argument that there's not much difference, because people are dumb, and I'd probably agree with you though.
Being a heroin addict should be treated as a social/medical problem and people given all the help they can to get clean, but we absolutely shouldn't be encouraging people to take it up. Especially heroin which is responsible for a very high percentage of all drug related deaths.