They are lumping tablet sales in with "PC sales", and showing them with a big fall (which is probably correct) So in their mind PC sales (desktop, laptop, "detachable tablets" which are not tablets they're laptops) will go up. They won't. They'll fall, and fall more than 1.7% annually they show the entire desktop+laptop+detachable+tablet market falling.
What's the incentive to replace a working desktop or laptop today? There is none, because the only major improvement made in that time is replacing hard drives with SSDs. The CPUs have hardly improved at all, so what was good enough in 2012 is still good enough in 2017 because Microsoft quit making Windows and Office demand more and more resources.
But being wrong is nothing new to IDC and Gartner. They have to be, because it is the PC OEMs, peripheral OEMs, software OEMs, etc. who pay them for the details on this research. People don't like to pay to hear something they don't want to hear. So of course their predictions are always too rosy, and always show huge growth in the latest trend (i.e. their ridiculous predictions of tablet growth a few years ago, now replaced by ridiculous optimism for what they call 'detachables')