It very much depends on what do you need a server for, and may not always be true.
However, IMHO primarily it's neither of these reasons you mention, nor any rational reason at all, but simply the fear of getting out of the comfort zone. The same reason why majority of people are not into DIY, but rather pay others to fix their plumbings or pour the concrete or change the lightbulb in a car. If you know nothing of it, running a server on your own sounds scary, you fear you'll screw up something, it's all stressful, and you'll rather pay to make it someone else's problem.
But then you also have a not-so-insignificant number of people who really enjoy in DIY approach and love doing as much as possible themselves. So the author's counterargument IMHO is more about Moxie stating something as an absolute truth, while in fact it's more like "majority of people will probably not want to run their own servers, under the current state of affairs". However I'm old enough to remember the time when the same applied for Internet - majority of people were not interested in messing with modem drivers and PAP/CHAP scripts just to connect to some BBS to chat with people, the idea seemed as ridiculous waste of time if you asked my father. And yet here we are now just some 25+ years later, whole world is connected. So perhaps the centralized platforms can't be avoided, or perhaps we need to make running your own server easier? And maybe both ways can co-exist, because different people want different things?