I have back up cameras in both of my cars, and I find them really, really helpful in many circumstances. Backing up to hitch to my small camping trailer is one where the camera allows me to line up perfectly with little effort. And backing out in a parking lot, where you can't see if anyone is coming down the lane as you start to back up the wide angle lens of the back up camera lets you see what your eyes cannot yet see by themselves. I wish there were a way to turn on the "back up" camera even when in Drive, for those times when I have enough stuff in the back of the vehicle to block vision via my rear view mirror. But I don't think I would want a camera to totally replace the simple rear view mirror. The savings in not providing the mirror is minimal and even if it were just for the rare case when the camera lens is really dirty, or the system breaks down, it would be worthwhile to retain the mirror.
As to improving mpg by removing the side view mirrors, while I would not want to lose those mirrors it probably does make a real difference. I have a Subaru Outback, and I read how Subaru engineers modified the very small antennae in the roof that is for the satellite radio system, and just by modifying this little item they added 1 mpg to the vehicle's performance. I would imagine the side view mirrors, if properly designed, could add a few mpg easily and probably make the vehicle look better as well.