It depends. With newer cars, you can design the car so that air deflects around the mirrors so that the mirrors may not create as much drag as older cars. e.g. the Leaf - its bulbous headlight design was not by accident. It guides the air around the mirrors to reduce wind noise and drag. Toyota has done something similar with the Prius but not as dramatic. Look at the top of the headlight cover near the forward-facing parking lights, you'll see a small ridge. Squat down and look at the ridge from the POV of air approaching the front of the car and see how it's there to attempt to bring the air up and over the mirrors.
I understand, but it's treating a symptom. Curing the disease would be to eliminate the side view mirrors. May on the next go around!
I agree. Did you see the Lane Watch feature on the new Accord? That may be our first step towards cameras replacing mirrors. They only thing is that a mirror is easy to replace. With a camera, you have the camera, wiring (and connections) as well as screen to go wrong.