Just to add onto what others are saying: to what extent do you think cops prevent crime? Most of what they do is catch people after the fact, and even then many (if not most) cases go unsolved.
I don't have the statistics in front of me at the moment, but the comparison that should be made is crime prevented by cops vs. crime that only exists because of cops, including things that cops just never get prosecuted for. I understand the impulse to think that cops are at least better than nothing, but I don't honestly know if that's true.