Earlier quoted context omitted.
As ericd says, a lot of those little flourishes actually aid human perception and cognition. Yes, we don't need giant flashy effects or lots of animation, but that doesn't mean that the most functional design is the most minimal. Also, it's kind've ironic that you can now have a 5k display to show off your flat, monochrome circles...
This. Shadows (fake 3D), textures, colors, and other elements may seem like a gimmick, but they are fundamentally crucial in visual cognition of the UI. What's sad is that so much UI design is driven by meaningless trends instead of more precise research data on how well UI elements work.
I'm sure there have been at least some regressions in some aspects of some user performance measurements, but it seems really clear at this point that it hasn't been a catastrophe… or even a significant problem.