What are marijuana
You seem to basing your argument on two things a comparison between alcohol and marijuana and secondly the ineffectiveness of incarceration. To the first point this is the fallacy of false choice. It's not a comparison of which is more damaging that is being debated. The debate is whether we should give legal status to an additional intoxicant. The question isn't proposing replacing alcohol with marijuana, therefore your argument doesn't address the question. To the second point decriminalization will not decrease use, however the criminal aspect may deter some. There is plenty of room for reform in the criminal aspect, and I'd be open to considering ideas with regard to that. The most damaging outcome is for society and government to accept and even promote use.