Yes, but civilizations and empires are not representative of the progress of humanity as a whole. Besides, isn't Dark Ages a rejected term by historians now? In addition, what is 200-400 years in the history of civilization? Of course there will be volatility - just like life on an individual level, the stock market, annual crop yields, etc.
I imagine that's what people said before the Dark Ages too. It's not as if empires and civilizations haven't ever fallen before.