You're not going to win. You can explain it to them, but you can't understand it for them.Watch the end of the video and tell me where the blind spot is. You can clearly see the vehicle in both the rear view and side mirrors as it passes.
They fail to realize that the idea is that without looking over your shoulder will see any car in either a) the rearview mirror, b) sideview mirror, or c) out the side window when you are looking at the sideview mirror.
The only thing accomplished by having the sideview mirror look down the back side of your car is helping make sure no one steals your gas cap.
But then again, those are the same people that will argue with you "the ocean isn't blue it's just reflecting the color of the sky"