It's also plausible that the 8% figure is representative of unfettered access. Conceivably everyone who wants opioids has access to opioids in some form or another now, and those who want it especially badly will put in special effort to get it. Heroin and kratom are easier to obtain than prescription opioids (if you don't have a visually obvious physical trauma and health insurance).
We know the number of patients prescribed opioids each year; it peaked in 2012 (it has been declining since then)[1]. We could look at abuse / addiction data over some period where prescriptions rose, and see if the disease rate rose significantly. It would be suggestive, if not conclusive, because we prescribe opioids to something like 20% of the population each year. (For comparison, about 66% of persons >12, or 55% of the total population, drinks any alcohol in a given year; figures from 2014.[2])
> After a steady increase in the overall national opioid prescribing rate starting in 2006, the total number of prescriptions dispensed peaked in 2012 at more than 255 million and a prescribing rate of 81.3 prescriptions per 100 persons.
I don't have good figures for historical rates of opioid addiction at hand, but we can look at 2016 (roughly).[3]
> In 2016, an estimated 1,753,000, or 0.7% of persons aged 12 and older, reported a substance use disorder in the past year involving misuse of prescription pain relievers
Going back to [1], the prescribing rate was 66.5 per 100 in 2016 and (in 2017, but taking for representative) "an average of 3.4 opioid prescriptions dispensed per patient." So we get 66.5/3.4 => ~19.6 per 100 people prescribed opiates, or 19.6% of people. 0.7% / 19.6% is an addiction rate of about 3.5%. There are definitely holes in this, but it's definitely plausible that the real rate for unfettered acecss is single digit.
We could also look at Portugal as an example. Decriminalization isn't the same thing as full legalization and unfettered access, but[4]:
> It is estimated that there were 33 290 high-risk opioid users in Portugal in 2015, which is about 5.2 per 1 000 of the adult population. In the same year, the number of people who inject drugs was estimated at 13 160 (2 per 1 000 people aged 15-64).
("High risk" in EMCDDA terms is injecting drug use or any long-term or regular use; it's a reasonable proxy for addiction, although might overestimate.[5])
[1]: https://www.cdc.gov/drugoverdose/data/prescribing/prescribin...
[2]: https://www.samhsa.gov/data/sites/default/files/NSDUH-DetTab...
[3]: https://www.cdc.gov/drugoverdose/pdf/pubs/2018-cdc-drug-surv...
[4]: http://www.emcdda.europa.eu/publications/country-drug-report...
[5]: http://www.emcdda.europa.eu/data/2014/methods-hrdu