> Shadows and subtle color reflections are a major part of that "layered paper" aesthetic.

They're a part of design language, raw elements of design. This can get much bigger than the Material Design of now. It can make sure that the subtle or even zany, domineering design of tomorrow has just the right level of refinement for its intended audience.

IMO as a graphic designer and as someone who has been doing 3D rendering as a hobby for 15 years, it's pretty awesome to think about PBR moving into the area of UI.