All very true...
Aside from punters holding onto their computers longer due to financial constraints, there's other factors at play here too.
Even computers 5 years old+ are entirely capable of running the latest version of windows (lets face it, that's what most average punters use). Lets not forget the legions of XP users still out there, quite happy with their old computers.
There's nothing exciting about the PC market, we may have gone from dull beige boxes to snazzy chrome, sweeping lines and multiple colours - but that fad has played itself out, as has the netbook, the last poster child of the consumer PC.
Punters want tablets and most of them want iPad's - that's where the new coolness is. They've got their aforementioned snazzy chrome, sweeping lined colourful desktops, laptops & netbooks - no need to buy a new one.
But tablets? The marketing shine is glowing strong.