I'm the author of the blog post so I'll weigh in too :-)
I agree with you @lucisferre, and for this reason I think the flat design trend will become obsolescent at a faster rate than most design trends usually do.
I also think that animation is going to become "the next big thing" and will be the primary differentiator between "meh" and "wow" apps.
I have by my desk Henry Dreyfuss's Symbol Sourcebook [1972 edition]. It contains symbols he collected over a lifetime - he died in '72. Rococo is the fad [Perlis says it comes before rubble]. Flat design dominates when clear communication becomes more important than signalling via fashion. There's a reason stop signs don't have gradient fills.
I think it's here to stay in some form. If you think of UI's as a language, it seems a natural simplification of that language. Simplification is a normal and natural thing to happen to a language.
I disagree. Language is about expressiveness, simplification in language is about reducing expressiveness which is neither normal nor natural, nor a good thing. Basically, thinking of UI as a language in order to make sweeping statements about simplification is a really bad idea.
I think we're talking about simplification in different contexts. Overtime symbols get simplified. That doesn't restrict the manner in which those symbols may be combined. Nor does it mean that, people won't embellish when they feel the need.
I agree with you @lucisferre, and for this reason I think the flat design trend will become obsolescent at a faster rate than most design trends usually do.
I also think that animation is going to become "the next big thing" and will be the primary differentiator between "meh" and "wow" apps.