Andreas gave a great response in a Q&A (30:08-34:24) to a question about centralization.
Summary: Today, bitcoin hardware becomes obsolete so quickly that it's only profitable if you use it for mining in the same country that it is manufactured. (Shipping and Customs take too long.) The reason for the high turnover rate is because ASICs have been progressing faster than Moore's law as they play 'catch-up' to the current limitations of nanometer scale.
But that catch-up day has arrived, as of roughly January 2017. It will take another 2 years before we see the impact – roughly early 2019.