malware.. and consumer lazyness / ignorance
"there isn't a compelling reason to upgrade a 3-4 year old PC anymore"
Whilst I'd agree with that statement for the average reg reader, for the average joe on the street after 3-4 years their laptop running windows is so full of bloat / malware and the disk so fragmented that it runs like an absoloute slug..
A mate of mine had a 2 or 3 year old acer laptop that he'd decided needed replacing since he'd picked up a virus.. I took a look at it and came to the conclusion that while his windoze installation was pretty much screwed to the point that it needed abandoning, a clean install of windows or any linux distro would render it fully functional again.. I offered to restore it for him (for free) but he decided he wanted to buy a new laptop anyway and I got a free computer.
As far as I can tell while his new machine has more ram, a faster processor, decent battery and is a bit lighter / shinier I doubt it really is £400 worth of improvement in performance on his old machine (with a clean non bloated OS) for surfing the web / emailing, skyping and watching downloaded movies on.
In a way I think microsoft is happy about the amount of malware on machines because so many consumers would rather just buy a new machine (and therefore new windows license too) rather than fix / pay someone to fix the one they've got.