Detroit’s population decrease has less to do with it’s crime rate, and more to do with it’s fading jobs market.
There was a time when Detroit had nearly 2 million residents, ranking as one of the largest cities in the US (2 million would be good for 4th place today, behind Chicago with roughly 2.7 million). The problem is that most of the economy in Detroit, Michigan, and the entire region… was based around manufacturing. In particular the automotive manufacturing industry (but also a variety of other industries).
Over the last 50 years those jobs (seen as manual labor, low education jobs) have been shipped overseas, or replaced by machines, or etc, etc. The jobs dried up in Detroit, and the population started leaving… Those that couldn’t leave were desperate cuz they didn’t have jobs, and were more prone to crime. What you eventually saw was an ever-increasing crime rate in Detroit, as the population became more and more desperate.
Detroit could very easily recover from this if they can get a steady amount of jobs to match their population. Look at a city like Pittsburgh. It suffered a similar (albeit much smaller) population/crime scenario. Pittsburgh at it’s peak had around 700k people. When the steel industry sent jobs overseas the city saw a spike in crime and a dwindling population. Today though, the city has stabilized. It sits at around 300k people, and has a very healthy economy based around various industries, and the crime has dropped significantly.