Pretty narrow view on the role of Law Enforcement.
IF (and that's a big IF) the rest of the system is doing its job, the prosecutors are prosecuting, Courts are handing down convictions when appropriate, Judges are sentencing violators commensurate with the crime, and incarceration keeps the perpetrators separated from the community, crimes are prevented--crimes which would have been committed by those in lock up.
Furthermore, the presence of LEOs in neighborhoods which might have crime, given that the deterrent of prosecution, conviction, and sentencing is working, can further deter those who might be considering committing a crime, simply because it increases the likelihood of being subject to the judicial system and paying a price for their actions.
Which means potential perpetrators are more likely to seek another means, a legitimate means, of acquiring the things they want in life.
Only someone who cannot understand that dynamic, or someone who has been accustomed to a system in which the system which backs the actions of Law Enforcement is broken or corrupted would think that police are only there to solve crimes.
That said, relying solely on that presence, even in the best situations, for self defense, opens the individual to being a victim. It's why we have locks on our doors, good dogs, and a .45 in the nightstand. At its most basic level, the citizen is the first level of Law Enforcement, by knowing and complying with the law, and by having the means and ability to keep the peace until authorities arrive.
