I remember in the 1980s and 90s when PC technology seemed to be improving daily, there was always a reason to get a better computer system:
(1) Processor too slow or obsolete.
(2) Replace tape drive with floppy drive then mini-floppy drive then hard drive.
(3) Hard drive lacking capacity.
(4) Move from text to graphics interface.
(5) Graphics card or graphics interface obsolete.
(6) Monitor too small.
(7) Move from 14.4K to 28.8K to 56K dial-up, then broadband.
(8) Move from Windows 95 to Windows 98 to Windows Me to XP.
Finally, in my world the "era of good enough" arrived around 2001. A machine of that ilk is good enough to run office applications and broadband Net stuff - even Quick Time with enough memory. I'm not a gamer so an obsolete AGP video card is adequate. When Windows 98/Me became obsolete all I had to do was boost the memory on my old PCs up to 512 Mb. That's plenty to run Linux or XP well. No need to buy a new machine to run Vista.
As long as my hardware holds out, I don't see the need to ever get a new PC. Even if it doesn't, I can search for a 2004-2005 era used one, or buy a cheap "ready to go" whitebox unit and install Linux on it.
It must really suck to be a mainstream PC maker these days. What can you offer besides a lame new operating system that really isn't needed?