Your anecdotal experience is opposite of mine: at my place, we still use 4-5 year old computers, and frequently upgrade hard drives, memory, video cards even when needed. We have a few rooms dedicated to spare parts, and when we can't find what we need, new ones are bought. And honestly, my 4yo PC doesn't really need upgrading just so I can open my IDE or browser 20 seconds faster.

Granted, we are an enterprise shop with a few dozen devs - exactly two people have a mac. So we're not exactly Apple's target (enterprise never was)