As I said, the studies are old. With the rise of superpotent synthetic opioids in the illegal market, and probably more caution in hospital use - one of the reasons these studies were made, was probably that someone noticed "wow, they sure used a lot of opioids during the Vietnam War, I wonder if that led to a rise in street addicts?" It's possible that it's no longer true that patients get much higher doses than the typical street addict. But it used to be the case, at least, and we can still learn from what we observed back then.
I think there's still plenty of support for the conclusions, that addicts can beat physical addiction, but that they start again, and that the fear of withdrawal pains is not a big factor in what's keeping them as addicts.