You would still have a major blind spot if a child or small animal is directly behind your car. The NHTSA estimated it would save over 70 lives per year (an underestimate), which is invaluable considering the small cost.
I appreciate the motivitions of those who think technology is the answer to accident prevention but now you are discouraging people from developing proper safety habits ("I don't need to check around my car because the cameras/sensors will tell me if I'm going to hit something") and then one day a sensor fails or they are driving a car that doesn't have this technology.
I already see this with my wife who has those little blinker lights in her side mirrors that warn if a car is next to you when you are changing lanes. She doesn't really look anymore, just makes sure the lights aren't blinking. I always turn my head and look; I don't trust the sensors and I also set my side mirrors to cover the typical blind spot.
https://www.caranddriver.com/features/a15131074/how-to-adjus...