It's not a patrolling problem or a plain clothes open carrying problem, at least not exclusively. Detroit is a place the gets most of its non tax income from down town activities and the drug trade, because that's all the suburbs see it as useful for. Guns protect individuals and distinct groups, they do not protect entire cities without turning into some form of overbearing police state, and even then it might not work.
It'd take a combination of better policing, extensive good samaritan OCing around the neighborhood, all kinds of other volunteer efforts, and thorough legalization of drugs to fix detroit. Even then you'd still have many thousands of kids with extremely bad situations at home who would be well beyond meaningful help.