The government's success at controlling people's alcohol and drug use has been dismal. About the only thing governments do is punish users and dealers, but that approach doesn't stop society from using either of them. It strikes me that America's war on drugs has been about as useful as prohibition in the early 1900s. While the wars strike me as failures, the taxes they consume seem to be going down the drain.
If legalizing marijuana turns out to be a social failure, maybe they can try something else other than law enforcement. I have no idea what that might be, and I'm not going to spend time considering alternatives unless legalization turns out to be a big mistake. But it's much too early to know how this will end up.
Personally, my decision will be to avoid recreational mind altering substances, but I don't have a need to dictate that to others. I doubt that marijuana is actually all that good for society, but I'm not sure how bad it is either.