For one, yes, I think there is a culture of people who do too much to play down the harmful side effects of marijuana. While it doesn't produce many acute symptoms, it fluctuates heart rate and blood pressure. Repeated use of chemicals which do that will take a toll on a person's cardiovascular system. Smoking anything, even if it's not carcinogenic, is bad for your lungs.
I agree that the full step to legalization is not the best idea, part of for the reasons you state. There are products like high content THC oil which can produce catatonic states, much beyond what natural marijuana would be capable of. Of course, there's not much evidence that this has overwhelmingly negative health concerns, but sociologically and psychologically this type of consumption for recreation is, all other things being equal, not desirable. And I do think people have not thought about this very much when advocating legalized marijuana.
This isn't reason in itself not to legalize, but I acknowledge they are things to take seriously and think about.
I'm not sure why you brought up the opioid epidemic as though access to marijuana correlates to opioid usage. Well, actually, they do correlate. It's an inverse correlation. States with legal marijuana correlates with lower opioid use. If people with real pain issues are choosing legal marijuana over opioid painkillers, then I think it's good that it's happening. That's true even if marijuana usage is not ceteris paribus desirable.
Otherwise, I'm not sure why you're picking a bone with marijuana in particular. There is a massive, massive, massive problem with addiction in modern society, and it's not limited to marijuana. Look up information on internet addiction (social media, phones, games), food addiction, gambling addiction, shopping addiction, or exercise addiction. Virtually any rewarding behavior has the capacity for addictive behavior. Some, like food addiction, are also driven by industry research and the social costs are staggering. Diabetes is estimated to cost the United States about $333bn/year. Other obesity-related diseases, such as heart disease, increase the numbers quite a bit.
It seems to me that allowing food companies to produce whatever kind of food they want and to produce disinformation about diet and health science are much more urgent than the social costs of marijuana legalization, when you consider that obesity kills hundreds of thousands of people per year. But, this is hardly ever brought up. Why? I think the reason this isn't part of the debate is because we have allowed extremely bad social institutions to become normalized. Marijuana, however, is new, so it's much easier to see the social costs. We remember life before marijuana, we don't remember life before companies marketed junk food to children.
All in all though I don't think legalized marijuana is a very big problem. I do agree with the concerns, but I'd fry bigger fish had I the choice. I don't see why I should be treated so hostile for thinking that and talking about it.