Wasn't trying to negate your point, just providing some information on the likely "still modern" state of iOS devices.
History thus far shows that 2.5-3 years is the point at which the older devices start to have issues, not being able to update their OS and therefore run apps. Hasn't been much history yet, though, so who knows.
It's a very young industry, which tends to mean lots of advances that are more likely to obsolete older equipment more quickly. Nearly all of today's Windows apps will still run on XP, and many of them will still run on Win2k. That's a solid 10 year span. Windows 1 couldn't (generally) run Windows 2 apps, despite being only 2 years apart, and Windows 2 couldn't run Windows 3 apps despite only a 3 year span.
It makes sense, I think, because the most innovative ideas are likely to come early in a technology's life cycle.
As I extropolated on the iPhone, the 3g and older models can't run iOS4. When iOS3 came out, Apple had mandated that all apps needed to certify to run with iOS3. This causes a push-out effect of the older devices. Sure, the apps you have will still run. But no new updates or innovations. Hence, the user will be tempted to upgrade.
Furthermore, as with older iPhones, the battery lifespan was about 18 months average. My 3g was good for the 2.4 years I had it, but I could tell the battery wasn't lasting as long. I assume the newer models have batteries that live longer, but still, as an embedded component, it's a driver towards replacement.
Additionally, as FL mentioned, portable stuff wears out faster. Plus people are hard on things. People drop their phones, drive off with it on top the car, let their kids play with it, have little regard for "it's raining and you're wearing $200+ in electronics".
Conversely, PCs sit around on a desk all day. Unless you're a gamer loading in the newest graphics hogs, you'll probably be using mostly the same software as when you bought it. Excepting the browser and its plugins (and even then, do you know how many rigs I see where they've never upgraded/patched ANYTHING).
Another odd trait I see is the casualness with which some people hop from carrier to carrier, phone to phone. To me, investing in iOS hardware is akin to deciding on windows vs. linux. I've got my whole NAS archictecure setup to support it. Switching vendors would waste money on apps and music that have been purchased. Yet some folks are buying new phones every year. there's a cultural effect that some people are driven to keep buying the newer version.