Maybe people are still using old PCs because they still work and are fully functional? I'm using a 4 year old ThinkPad and guess what? It's not only still fully functional, it's still quick, snappy, the screen is still bright and looks nice (I did splurge for the upgrade at the time), and it's been super low maintenance (been running Linux on it the whole time). It could use a new battery (capacity has gone down over time), but hey, it's a removable battery so I can do that.
I imagine I'll still be getting a few more years of use from it, there certainly are no signs that I should upgrade at the moment. I mean, if I were to be a little greedy, I'd buy a new laptop with a wicked video card and give this one to my wife (after she made me throw out a fully functional 6 year old desktop we didn't use very often), but is that necessary, not really.
It's pretty plain that Apple simply plans for its products to be replaced quicker than 5 years, to make more sales, and they're speaking to the faithful.