Mu guess would be that iPad sales are down simply because a) hundreds of millions have been sold already, b) even more competing (primarily Android-based) tablets have been sold, and c) contrary to smartphones, tablets aren't typically bought on contract and replaced every few years, and they don't add nearly as much utility as a smartphone for someone who didn't have one before.
These 3 things combined indicate the market for tablets has been nearly saturated, and the majority of people who want one (and can afford it) already have one. The iPad has moved from 'growth market' into 'replacement market'. I know many people who are still using iPad 2's or even the original iPad, for example my mom still used the iPad 1 I gave her when I upgraded myself, so I'm not surprised Apple doesn't sell nearly as many iPad Air 2's as they sold iPad 1 or 2.
The article itself is pretty dumb if you ask me, and far too hung up on the truck/car analogy, and the premise that a device can only be useful or enjoyable if it replaces some other device like a phone or a laptop. I have a smartphone, tablet, laptop and a fixed desktop at home, but if my tablet died I would get a new one the same day. My laptop and desktop are far more likely to be replaced by a single device than my laptop/tablet or smartphone/tablet.
Just looking around I see iPad's everywhere, all the time. I see people who hate computers but love their tablets. The idea that declining sales imply they don't have a use is quite ridiculous.