I'm not sure where you are getting your estimate that 10-20% of the population would be addicted to morphine if most drugs were legalized in the USA today. If you look at the stats on Portugal, you'll see that drug use and drug related deaths have declined since the country legalized all drugs[1].

Also, morphine was only invented around 1818 and its addictive properties were not understood until much later[2]. For example, "...initially morphine was also marketed as a non-addictive cure for addiction to both alcohol and opium..." That was happening in 1853, nearly 40 years after the invention of morphine.

In the article you linked to, the author states that many of the morphine addicts of the 1900s were people in rural communities who were sold "elixirs" by traveling "doctors" and these potions were often 50% morphine. Which causes the author to state: "...if you look at drug addiction in 1900, what's the number one way in which it is different than drug addiction today? Answer: Almost all addiction at the turn of the century was accidental."

If drugs were legalized, consumers would not be getting accidentally addicted to morphine as they were in the 1900s, because they would be able to know for certain what drugs they were buying and ingesting.

[1] https://mic.com/articles/110344/14-years-after-portugal-decr... [2] http://www.todayifoundout.com/index.php/2014/09/invented-mor...