I know very few people that constantly upgrade their PC builds. The ones I do, upgrade for the sake of upgrading, not because the hardware is no longer able to keep up. For example, a new minor version of a video card comes out, and they drop $1000 (AU prices.. sigh) on upgrading the card. They might do that 2 times per year.

Most of my friends that build new PCs will do so every 3-5 years but, like you said, very little is salvageable. Non-OS storage being the main reusable component.