Policing does reduce crime. To see that you should compare crime rates where there is more policing with crime rates where there is less policing.
Seeing 10 robberies ("crime is thriving"), you conclude that "police don't prevent crime" because that number is not zero. The logic here is incorrect: you should compare 10 not to 0 but to the number of robberies that would have occurred if police were not there --- 12, 14 or some other number, which we know to be greater than 10.
"As I said previously, police don't prevent crime. Why? Because the criminals don't commit crimes when the police are around."
Please talk to someone or read something on the mechanics of law enforcement. I cannot contribute further to this discussion.
The same bottom line applies: They arrive at the scene of a crime after the fact. They investigate after the fact. They look at the crime scene after the crime has been committed. They pick up the bodies after the murders.
If you think the police are protecting you, you are living in a dream world.
Good luck!