..you know, rear defrosters are a BIG battery drain.... for BEV, skip it. I wouldn't care if my Model S had omitted the feature, I would still buy the car. I never use the button. And I live in frozen Canada!!
You don't even need to have a rear window, by law. So long as you have two side mirrors. Tint it all you like. Replace it with metal.
For frost / ice... using battery power is expensive, use a scraper outside instead before you drive.
Ask yourself if looking out the rear window clearly is a real needed? Or if looking through it foggy is 'good enough'. I let it fog up... I don't care. I can see headlight glare through it at night, or dark shapes in the day if there's a car behind. That's enough information. I'm a forward driver... the front glass is uber important, ..the rear, meh?
Tailgater? Hit the "go pedal" and anything behind you is now further behind you. Guaranteed. That's the beauty of these cars, you can just "buy" room in a couple seconds.
Use the rear camera if you want to see a little more back there. If something really interests you, like the wall you'll hit when backing up.
I would NEVER depend on rear window only for backing up the car. I would get out and clean the camera if it was mucky / foggy.