I bought this Dell XPS 8300 about 4 or 5 years ago... and it still does everything I ask of it, without issue. I upgraded to SSD, and have gone through a couple GPU's, but the system itself is a rock. Of course you're going to see a decline in sales when systems you buy today aren't really any better than ones we were getting 5 years ago. Feels like we've hit a ceiling or something for desktops. And currently, it's not an issue, cuz what we have is running what we want without issue. Age old saying: If it ain't broke, don't fix it.
Throw smartphones into this mix, which provides a pretty impressive computer in a very small package, and many 'commoners' find the smartphone does everything they need, in terms of a personal computing device. These people not only don't want desktops, they don't need them. My phone runs some stuff just as fast as my desktop computer does!