As you say super flat might not be great, but all the designs preceding the current trend where extremely harsh on the visuals.
From windows XP to 7, the windows default themes and colors where disturbingly flashy and "in your face". OSX was more bland but aqua was still kitch and unneedingly strippy. iOS6 app's ultra textured or "realistic" interfaces were the peak of that trend IMO.
In the real world, not everyone likes kids playroom colors, nor grandma's 60s style wallpaper, nor steam punky design, nor green-blue metalic robot parts. Personally I like MUJI style clean and clear design, and I feel like it took that much time to have a core of people to value cleanness and simplicity in computer UI design as well.