The story to which you link notes that "The impact for 2016 will be larger as detachable tablet volume grows, boosting earlier forecasts of PC growth in 2016 from -3.1% to growth of 1 to 2%."
The quote is saying that if you add those "tablets" back, it's predicting that the PC market will grow this year.
Another important point is that the vast majority of Windows 7/8/8.1 PCs will run Windows 10, and the upgrade is free. There's very little need for most users to buy a new PC.
If users only buy a new PC every six years, then Windows PC sales should halve, compared with a three-year upgrade cycle. It doesn't mean that PC usage is declining (though it might be), it just means that PCs are lasting twice as long.
Smartphone sales were, of course, hugely inflated by the fact that they only lasted for 18 months or so. When people keep their smartphones for three years, smartphone sales with halve as well.
https://www.asus.com/2-in-1-PCs/ASUS_Transformer_Book_T100TA...
There were 275 million PC units sold in 2015, and approximately the same number is expected for 2016. Sales were growing until 2011, when they peaked at around 360 million units. So certainly sales have shrunk, but I wouldn't describe them as "shrinking quickly", according to forecasts.