Also indirectly is the Microsoft stipulation that laptops with Windows 8 are to include touch screens, so now all the low end ultrabooks, which were becoming mainstream, are now crippled for screen image quality, due to the compromise on screen quality to offset the additional cost of the touchscreen components. Most people refuse to move up another $300 to get what they could with Windows 7 6 months ago, and they're certainly not buying the low end rubbish.
As stated above, processing power in desktop machines surpassed the vast majority of users requirements 5-6 years ago, and apart from power users and gamers, no one actually need to upgrade their desktop pcs, unless something breaks down.
Even on the gaming front, requirements have become stale as games are developed for the lowest common denominator (10yr old consoles), and PC games are an after thought.
Monitor resolutions have stagnated, cpu requirements have stagnated, the only progression is in storage, and that's a trivial retrofit for most desktop machines.
Recently, my workplace has started swapping hdds for SSDs in 3yr old machines, rather than replacing entire computers (except servers). My i7(v1) work pc just recently was retrofitted with 8gb ram, 256gb ssd, 3tb storage drive and Windows 8 (because I'm the guinea pig) keeping everything else.
My personal pc has a 4yr old cpu + motherboard & the only recent additions are video card, ssd and bluray drive, and I'm a nerd!
When the nerds stop upgrading, the pc industry is in trouble.