well of course pc sales have gone down. Why by an expensive gaming pc when you can play most pc games on cheaper consoles without having to upgrade every 6 months?
You've completely missed the point. This is general PC sales, not just gaming PCs or even gaming capable PCs. It's not really related to PC as a gaming platform as it's actually a relatively sucessful niche (niche compared to the rest of the PC market) at the moment with Steam and other digital platforms each serving a selection of downloadable games ranging from small indies to AAA budget titles.
This decline is more related to general consumers and businesses not looking to upgrade their hardware. It's not that surprising considering most PCs now will last over 5 years quite easily for most business tasks and tablets/smartphones are more than capable of general consumer tasks like web browsing or basic multimedia features like video.