Read other letters about this article
DanielM, you make two very good and important points (that the difference in resale value depends on what time frame post-purchase we are considering, and that most people don't resell their computers after a short number of years). A lot of people have been saying "but people don't resell their computers!" but I want to make that argument clearer, because maybe Manjoo's point is that people should resell their computers. So more on that below.
But first, I want to say that I am open to Manjoo's main argument (that Macs are cheaper than PCs in the long run because they retain their value better for resale), but the data he presents is much too simplistic and anecdotal to effectively defend it. The situation is so complicated that the data would be hard (though not impossibly hard) to compile, and any generalization (such as the one he makes) is likely to turn out to be false. One would have to take into account different brands of PC, different price ranges and tech specs, and different amounts of time to resell. But anyway, as far as that goes, it is an empirical question and one that could be answered through research.
However, there is also the probably fatal flaw that, even if it does turn out to be cheaper to buy a new Mac and resell your old one every few years than it would be to do that with a PC, it's still got to be more expensive to buy and sell your computer every few years than just to keep the one you have until it doesn't work anymore. So if we're looking for what is cheaper, Manjoo's path doesn't help.
As an example, I recently bought a new computer to replace the one I had for the last 7 years. I did so because the old one broke and I can't fix it, so of course I won't be reselling it. I could try harder to fix it and then resell it, but if I could fix it then I still wouldn't sell it because I'd be keeping it instead of buying a new one. Most people (who aren't tech bloggers) can't buy a new computer every couple years just because it's convenient or cool; they only do so when they need to, and that's because their old one is a junker (which probably has close to zero resale value).