A good friend is a retired CHP and he has told me many times that when he was trained they were taught to stop the person, once they were stopped, they stopped shooting, which goes by your statement of stopping the threat, but todays officers are taught to execute them and basically don't stop shooting until they have to reload and that is not my words but a retired CHP officer.
As to not every shooting being a textbook example or done to textbook perfection. It should be because they are trained to do it right and are paid accordingly to do so. Yes, not all people should be cops, but once they are it's tough to get rid of the bad ones because of the brothers in blue and cops protecting cops. Furthermore, to say that most cops shootings are justified insults most peoples intelligence, a good portion of them are probably not justified. Today, in the Bay Area, police shot and killed a man because he had a butter knife and the witnesses and there were many said he was posing no threat to those around him or to the police, but when he didn't drop the butter knife the cops shot and killed him.
Finally, I have only been able to find the number of people shot and killed by police in the US for 2015, is over 1,200 or over three per day! I have been looking but have been unable to find out how many they shot and lived. We do lead the world in the number of police shootings per year and we lead by a wide margin.