Flat is just so... flat. And boring.
And also harder to read, because it's harder to orient yourself in a big textureless area than in a space with some texture in it.
https://arstechnica.com/gadgets/2013/09/ios-7-thoroughly-rev...
But the idea of making a subtle signal that this thing you’re interacting with will have external effects, in the age of MP3 players and digital cameras, was smart.
Notice the shadows inside of the in the inputs, and the 'shine' applied on the bottom. Web designers from the 2000s will remember all this stuff. What's old is new again!
In my opinion, OS X has been (mostly) downhill after that. The iOS-ification began with Lion, which changed the juicy blue scrollbars to a boring grey, got rid of 'Save-As'—an extremely controversial addition, as I recall it—and added then-newfangled things like iCloud.
