You also hold the unit quite a lot. The feeling matters.

Yes, it does, which is why the iPhone 4 was a terrible design. Pull the phone out of your pocket, and there's a 3-in-4 chance that you're "holding it wrong" right off the bat because the front and back feel the same.

No, thanks. Every surface on a phone needs to feel at least subtly different.

The glass front and the metal back always felt different to me... and in any case, the very first thing I did is look for the home button with my thumb.

I haven't upgraded since iPhone X. That one doesn't feel different wrt front/back at all. Does iPhone 11 feel different in this regard?

Yes, the 11 has kind of a “matte” slightly textured back. It’s definitely easy to determine which is which just by feel.

This is still a one time design cost though. And nothing about it means that you should iterate the design every other year. Materials maybe.