His overuse of the exclamation point aside, I don't think he was saying it's stupid becuase technology makes it possible. He was saying the vast majority of Apple's (and indeed the computer industry's) users will not be interested in installing another hard drive, video card, DVD drive... even adding memory is iffy for several. These users (to his original point) are not the vocal minority but the silent majority. I strongly concur with him. I've converted 10 families/friends and exactly none of them have asked me how to replace anything in their 2-4 year-old macs. None of them have even mentioned it as a need.
Apple probably knows this and they've designed the iMac around it. It was the spirit of the original iMac and remains its spirit today. To say that Apple hopes the users might buy a new iMac when they need a new hard drive is a stretch. I think it's more likely that they want people to buy a new iMac when they want a new FEATURE (larger screen as an example).
The 24" owners you mentioned, who are pro users and casual gamers, would in most cases be better suited to buy a Mac Pro as this will give them better value (this is why I got one rather than an iMac). They know it and Apple knows they know it. The percentage that you (very correctly) say they will lose by setting it up this way isn't as important to them because they are out to serve the silent majority.
Just my opinion as I see it.
rjf