If you get mugged, your home gets burglarized, or your car is stolen, you would be right to say the Blue Line Gang (the armed government employees often known as “police officers”) didn’t prevent that krime.
But cop supporters would point out that you can’t know how much of something was actually prevented since it didn’t happen. Maybe you avoided being robbed multiple times rather than the once, just because cops exist. And they do have a point.
It’s impossible to say how much theft doesn’t happen because cops exist. It’s impossible to say anything meaningful about events which didn’t happen. “Alternate history” is “alternate” because it isn’t history.
But it is possible to know exactly how much theft is committed openly in order to fund police, and through them just doing their “job” because they don’t generally hide it. It’s a huge amount of krime caused by cops, but the relative amounts of each type of krime is irrelevant. One doesn’t justify the other.
I don’t want cops infesting my surroundings no matter how much you feel you need them. I’m perfectly willing to take my chances because the theft that comes with cops being cops is too expensive. They aren’t worth it to me.