Well that's the thing isnt it... the desktop pc *isnt* in decline, its simply that other devices are now available and becoming mainstream. It may account for a smaller percentage of the market than it once did, but it's no less popular than it's ever been. A hypothetical 20% is still a nice chunk of the market. If the remaining devices (phones/laptops/netbooks/nettops/laptops/other consumer devices/etc.) take up 80% then the desktop computer is more than holding its own.
The sooner all the consumer driven drivel migrates to the fad devices the better in my opinion.
As I already said there's some tasks that are simply impractical on anything but a fully fledged desktop and there's still plenty of room for needing more computational power, even if some people seem to want to believe otherwise.