The article has input from Gary Glass, so they did some research. As Denny said, this has been a topic, covered at length in the GC meeting.
The AHA growth has slowed, but was still slightly positive or flat last I heard.
The revenue at retailers is down. Less high margin extracts are sold as BIAB becomes more popular. Less fancy equipment sold for the same reason. Less beer is brewed due to demands on time (work, family, and so on). Every town having a brewery (it seems that way) cuts into the need to brew fresh beer.
There are many new devices coming out, the pico brew and grain father systems, that automate brewing. Those may bring more people into the hobby, as the automation has an allure for some.
Things change. Double digit growth doen't last forever. I think the hobby will be fine in the long run.