Your assertion  opioid users are addicted predominantly 'cause they're mentally ill or traumatized is dubious. Though some are, people who become physically addicted to substances are not necessarily mentally ill or traumatized. If they were, in order to be consistent, you'd need to also claim most people who are physically addicted to caffeine, nicotine, alcohol, and even junk food are mentally ill too.  Huh?

Was it mental illness that caused heroin addictions of Keith Richards, Pete Townshend, Eric Clapton, Jimmy Page, John Lennon, Ray Charles, Billie Holiday, Charlie Parker, Miles Davis, Thelonius Monk, Boy George, Slash, Debbie Harry,  Johnny Winter, Gregg Allman, and myriad other celebrated rock and other stars?
Doubtful. 

Addiction is hardly a one-size-fits-all phenomenon, the "one size" being the false claim addiction is essentially just a symptom of mental illness/trauma. It's not nearly as simple as that.