We can get obese people to regain their health. We cannot restore the brain to its state prior to the hastening of psychosis and we cannot recover IQ points. We know that starting THC at a young age does those things and more.
Beyond that, I don't think fast food is the primary culprit for obesity, diabetes, heart disease and the like.
I used to argue the same things about the black market and how it would wither away with legalization. It hasn't happened. In those states that have gone down this path, the black market is thriving. It turns out the this piece of gospel hasn't been borne out in our various state laboratories. One unintended, excuse me, intended consequence has been an increased focus on getting more young people as THC users. This is no different than what the tobacco companies did, and they will be jumping on all fours into this industry soon enough.
I don't get into the addiction debate when discussing drugs too often. We have both known people who are not "addicted" to alcohol, yet their consumption causes problems in their lives. Sometimes, grossly destructive problems. I'm less worried about THC than heroin use, for example. Contrary to what is shown on TV and movies, heroin is a less difficult drug to kick than many others and THC has greater long-term ramifications for long-term users. Additionally, its use hasn't been normalized like THC has been.