An ultrasonic flaw detector is a clever instrument. It works on the principle that sound waves are distorted when they encounter a crack in the metal. When the detector is beamed through a defect, its sound profile changes and alerts us. Once we detect the fault, we fix it.
Cynical people perform the same function in society. Cynical folks are usually intelligent, and channel their smarts to look for faults and cracks in society. Once they spot these faults, they are only too vocal about how exactly the world is broken.
Once a cynic alerts you to a fault, thank them and go fix it.