Addiction is not illegal; the act of procuring and using drugs illegally is. Actually, what you're describing is called "doctor shopping," and obtaining drugs that way is illegal and can be prosecuted.
Are you asserting that the major harm drug users cause society is the act of enriching street drug dealers and their black-market enterprises? I.e., if someone is abusing drugs procured from the pharmaceutical industry, it's much less harmful to society as a whole?
Yes.
Some may take this to mean that we should end the war on drugs and let the corporations manufacture consistent quality drugs.
Would we even have prescription drugs then? If marijuana, cocaine, heroin, and lsd become as legal as alcohol and tobacco why would someone need a prescription for oxycontin?
If oxycontin still proved to be a better reliever of backpain than heroin (or did not have the same health/withdrawl factors) would there be some people seeking pain relief by illegally buying oxycontin without a prescription?
Unless we are willing to legalize all drugs (including medicinal and including doing away with the FDA) there will always be a war on drugs.
The purchase of street drugs does finance a lot of criminal enterprise (from bribing public officials to gangsters defending their territory to some paramilitary rebel/terrorist organizations). Also organized crime exists to fill the niche markets. If they don't traffic in drugs, they will traffic in prostitution and pornography; there are also payoffs between organized labor and contracts, murder for hire, gambling, and all sorts of enterprises (including "protection money" shakedowns).
Ending the war on drugs won't spell the end of the mob. Just like the RIAA keeps seeking out new sources of revenue to fill the gap caused by shrinking markets (the baby boomers no longer buy as much music as they once did), so organized crime would continue to keep revenue at current levels or higher.