We shouldn't forget that these are all iterations on existing designs, and that at least partially the improved readability/understandability(there's a better word that is escaping me here) is in part due to this in addition to the flatness. It isn't just "flat" that's doing it here, a lot of these examples are also making better choices with whitespace and data presentation in addition to downplaying gradients (chatter moving to cards and ditching the tabbar for example).