You will get the standard barrage of false logic, claiming that a mirror cab be replaced with "turning your head". This hopelessly fake logic ignores the simple fact that turning your head is a conscious action, which you have to initiate yourself. This alone already negates most of its usefulness: unfortunately problems often catch up with us at those moments when we simply didn't know that we needed to "turn our heads". If the cyclists had this sort of divine foresight, the number of car vs. bicycle accidents would've been at least a million times smaller.
The point of the mirror, on the other hand, is to be present in your peripheral field of view at all times. A mirror, even a small one, very reliably warns a bicyclist about a car approaching from behind in situations when the car is easily discernible on the background. It works especially well for cars with daytime running lights. This sort of detection requires no conscious action on the cyclist's part, which is the whole point of a mirror as a warning device.
Insisting that it can be somehow replaced with "turning your head" is an obvious and rather laughable demagogy. The only way to achieve the same level of backward awareness by "turning your head" is to look backwards at all times. Needless to say, this is not the healthiest way to ride.
The only thing that can replace a mirror on a bike ridden on public roads is that proverbial magic "crystal ball", which can tell when you need to "turn your head". Unfortunately, the don't seem to make these in any noticeable numbers.
