Because if the case gets damaged you can easily replace it.

People would still put a case on a bulky phone to protect resale or trade in value.

A super thin phone doesn't require a super bulky case, it requires just as much case as a person would normally use, resulting in a smaller overall profile.

I'd probably still go pro because I care more about the camera than the size.

They're sort of doing that with Ceramic Shield, and the new bumper case for the Air.

I don’t get the idea of putting glass on the back of this phone. Wouldn’t it make more sense to just have metal there so there are less things to crack?

Metal interferes with wireless charging.

Another alternative is hard plastic, but it doesn't "feel premium"

I think the reality is that your phone will always encounter something that can damage it (unless it's made out of diamond), and a case lets you easily replace the damaged part.

Just subjectively, I remember having a super scratched iPod and it just felt kind of ratty every time you looked at it. Meanwhile, a phone in a leather case gets kind of a patina that improves with age. It is kind of sad though, I got a really pretty blue iPhone and you wouldn't even know it because it's completely covered by a case.

I'd rather see phones that aren't perfectly flat but instead have proper built-in mounting things. Give me some screw holes to attach a set of 4 bumpers to the corners, and a camera mount thread hole in the bottom.

Honestly, it sounds really compelling to me. I do a lot of stuff outside - search and rescue, climbing, etc, and I need a rugged case for that, but having a thin, light phone when I'm at home or doing something with less chance to damage the phone is pretty nice, so thin phone + case is the best of both worlds.