I have seen a couple rehabs out there that are designed around the idea of converting full-blown addiction to responsible use. But your idea, that is the idea of taking controlled use and simply extending it indefinitely, is not something I have seen.
What I find interesting is the concept that alcohol, which is an incredibly addictive drug, is actually used responsibly by the majority of its users. The first question that comes up is along the lines of if alcohol were freshly discovered and being sold on streetcorners or the darknet, would the rates of addiction be higher? Would it be that much harder to not get addicted to it if society treated it just like any other drug? On the flipside, if opium dens existed in cities, could people go to them on the weekends just like they do the bars? I'm trying to get at something like an intrinsic addiction rate for different drugs that exists beyond all of the varying social in/acceptances and such. While it is easy enough to say "controlled opiate use...ha," I do think that some people could pull it off within a very different culture.
In this culture, though, I think there will be legal reasons why no treatment facility or outpatient program would want to go this route. And then I think a lot of people would be hesitant to support this, as it could lead people into addiction by a few successful examples being spotlighted and giving false hope.
I do think that there are very real addictions out there that are forced to take this approach to try and help people, though. Eating disorders - you still have to eat. Shopping disorders - you still have to buy stuff. So maybe a good starting point is to look at how people in these types of addictions are shown structure, rules and discipline for moving forward more sensibly.
I quit hard opiates but now use kratom in a controlled way, so I can relate. One beer and it would all be over. One line and it would all be over. But I can do kratom once a week, some weed twice a week. Some people know that they are in the all-or-nothing crowd, but it's wrong to assume that all people belong to the all-or-nothing crowd. Trial-and-error is really the only way to find out what you can or cannot handle.