Let's break this down.
The crimes that are most destructive of human well-being are sexual assault and family violence (spousal, child, elders). Police contribute in no way whatever to the control of these crimes. (The police-prison system _may_ increase their fsrequencey.)
Various traffic violations are probably the second most destructive from of misbehavior. An unarmed police force, not empowered to control crime, might do a far better job of controlling traffic.
Serial killers do exist, but there is no evidence that armed police or prisons contribute in the least to the prevention of these c rimes. Were the Myth of Police Protection eliminated, more thought could be given to reduction of such violence.
It is hard to identify any kind of serious crime which is reduced by police & prison. The elimination of criminal justice would create greatly to more constructive thought on the reduction of crime.
Three of the greatest criminal organizations in the U.S. would be immediately disbanded by shutting down armed police: The New York, Chicago, and Los Angeles Police Departments. Public safety would also be greatly improved by the elimination of immigration control
It is difficult to see _any_ servie that Police Departments and Prisons actually provide.