Except a hundred years of experience has shown us that "regulating drugs until they're no longer in existence" is not possible. What actually happens is drugs become more expensive and more dangerous. You bring up legally prescribed opioids, but those are drugs that people take because they're encouraged to do so by a doctor. Your mother most likely wouldn't have taken Fentanyl if it was just legalized, but not prescribed to her. Most people who would take legalized drugs will also take them when they're illegal.