This is nothing new. As long as there have been GUIs, platforms have issued style guides to developers and expected them to emulate the look-and-feel of the platform's own OS and flagship apps. Remember "lickable" OSX?
Flat itself is a design fad like any other, and the reality is the app's appearance and adherence to trends is a real signal of quality to most users.
> As long as there have been GUIs, platforms have issued style guides to developers and expected them to emulate the look-and-feel of the platform's own OS and flagship apps.
There's a difference between having a platform-specific GUI/style-guide (which I support), and changing it every few years for no apparent reason (I understand e.g. the change from Apple OS 9 to OS X - they were simply catching up with the hardware improvements - but OS X to OS Y(osemite) seems quite arbitrary).
Flat itself is a design fad like any other, and the reality is the app's appearance and adherence to trends is a real signal of quality to most users.