Your looking at the market wrong
The market for selling PC kit might be slowing down, but market usage of PC's is not. The fact of the matter is that anyone in the developed world who wants a computer has one. The market has reached saturation, and it has been at that point for a while. In the past, despite market saturation, people regularly upgraded because new programs required better hardware to run. However, that is not the case anymore. The latest AAA games can all be played on 3-5 year old Hardware (unless you absolutely needed every graphical setting on max). So the Need to upgrade is gone, and now the replacement cycle is much slower. The market is not going away it is merely recalibrating to the new reality where new Hardware does not offer the significant increases of previous generations.
Comparing this to Smartphone and tablets is not a like for like comparison of where the market stands now. Every new generation of tablet brings significant gains in usage, power and abilities. As such, the upgrade cycle is fast and it is selling well, eventually it too will hit the same saturation and Performance drop off curve but it hasnt yet.
Smartphone sales are dropping back (just look at Apple's recent sales show) and the reason is that they have now hit that saturation point, each new generation of smartphone doesnt really bring any huge benfits, the phone is only slightly faster, with slightly more memory, slightly less weight. And its safe to say everyone who wants a smart phone by now, has one already. The upgrade cycle is now getting longer, new phones are only purchased when old ones die rather then on the previous regular basis. This is just the new reality.
The current market realities were always going to occur, market saturation was inevitable, we should be congratulating the PC firms for getting a PC in almost every household, but they should never have expected to continue to sell us a new PC every year, if they werent able to maintain the massive Speed increases that fueled those yearly PC sales...