The notion of flat design has seen some tremendous success in the last years (even Google is doing it). There is an abundance of sites and blogs that will tell you what flat design is and how it will revolutionize design, especially interface design. Some see flat design not as a form of style but as an inevitable development.
Flat design is as a form language and style quite interesting especially after a long period when digital technology made it possible to imitate reality in all its glory and richness. So, it is not strange that there is an attraction in the opposite, which some see as a turn towards the "real" qualities of digital material (which makes the whole question more philosophical).
Anyway, all this is fine and well. However, we will all quite soon be bored again, and flat design will be seen as "old" and maybe "flat". I have no problem with that, that is the way style and fashion in any field works.
What is somewhat problematic is when designers argue that flat design is not a choice, not a style, not one out of infinitely many possible ways to design. When they see flat design as a culmination of an evolution or as the unavoidable paradigm. Such a view is highly unfortunate. It leads to simplistic understandings of design and at the end of the day, it leads to a narrowing and streamlining of the design space which in turn lead to a deprived design landscapes and ultimately boring designs.