You're a lot less likely to relapse and commit crime when you're already on a stable dose of your drug of choice. Considering how many people in america are on legitimate opiod prescriptions I find it insane how people instantly jump to 'well you wouldn't legalise heroin would you?!', there's no meaningful difference between opiods whether they're prescribed or not.

Why concentrate on anything? Heroin addicts who desire treatment will be welcome to it, but others will choose otherwise.

One argument would be that taxes from the sale could be used to fund better addiction treatment than is currently available. State sanctioned supply would then be taking money out of the hands of organised crime and using it to help the people who need it.