This is not about Windows 8, this is about the web - the web is killing new PC sales. This trend started ever since Windows XP which coincided with the great shift of the net from a publishing platform to an application platform.
Simply put, we don't need newer and faster computers to use all the web sites and web-based apps that we know and care about. Even gaming hasn't been pushing the need for people to upgrade as fast anymore.
This is not about windows 8, it's about the maturation of the computing industry as a whole. We current needs most people have for computers is still well served by 5 year old hardware.