I don't think this is true? As far as I can tell any time the recording is mentioned in a complaint at the police behaviour the camera was off due to [battery life|maintenance|other].
You also get just rank intimidation. My friend got out of jail and the next day tried to file a complaint about an officer stealing his pocket money during an arrest. He was placed in a room with the arresting officer, who explained that any complaint which was filed would incur retribution from the DA with regards to the thing that got him arrested. This is happening in a large urbanized precinct, in a blue state.
To ever regain control of the police force, the various civilian & political oversight bodies would need to prosecute thousands of felony exortion, kidnapping, and assault cases a year.