No one factor determines anything
To say that the iPad is killing PC sales is trying to say that one single event, or one single word, started World War 2; there were a lot of mitigating factors that came together and affected the overall timeline of events, resulting in what occurred.
The tone of the article leads me to believe the comparison is between iPads and full, out-of-the-box unit sales, which is a false correlation as it doesn't address all the variables.
I agree with richcobrien, in that many people aren't moving away from their established PCs because they still work and the economy dissuades new PC purchases and instead pushes cost reduction processes that don't usually entail such an expenditure. This can range from changing a business' network layout to better utilise virtualisation or cloud technologies, or a single user adding or upgrading a peripheral or internal component.
An iPad is bought because it fills a need, whether it be perceived or real. It also fills a niche that PCs don't cater to. However, so do a lot of other tablet PCs on the market; why isn't this article titled "Why Tablet PC popularity is slowing PC sales"?
I'm writing this on an 8-year-old Dell Dimension 3000. If I max out what I can do to improve the performance (a ??50 graphics card is in the works, it already has a 2GB RAM upgrade and an extra hard drive), I'll be battling with my employer ("why do you need a new one? We haven't got the money") to replace it with another desktop PC, not an iPad, because it'll be what I need to get my work done - and there are plenty of PCs out there which do the trick for less than an iPad.
