People aren't going to agree about this, but this is my view.

On its face, the war on crime seems clearly to have been bad for Detroit. It didn't succeed in keeping crime at a reasonable level [[in fact, crime got substantially worse in both Detroit and the country for many years afterward), but it locked up and thereby economically and socially disabled a big chunk of the population, with various additional resulting problems, so in my view it was minimal upside and lots of downside. In that sense, I would say the professor is correct.

Of course, we don't know what would have happened if different policies had been pursued--perhaps crime would have reached levels well beyond what they actually did and that could have been even worse. On the other hand, you can easily imagine better criminal justice policies having been implemented which could have both resulted in better crime control and less social disruption, so in that sense I would say you could certainly fault the actual implementation of the war on crime.

How important this was in the constellation of factors behind Detroit's problems is probably pretty unknowable. A big chunk of Detroit's decline had already happened before the war on crime really started, so there is a limit to how much blame I would think it should get.