Well, I guess fewer and fewer folks would buy your products if they kept breaking! My take on Apple, in regards to buying new replacements for their phones goes like this: they roughly planned (or hoped) folks would use them for about 2 years and then replace. But not because they were no longer working, indeed they make a pretty good product (both hardware and software). What they did is consistently make improvements and added new features that folks liked (or were made to believe they liked). Whether it's a little bigger screen, siri, better camera, pictures that briefly animate, better processor, pressure dependent responses and so forth. So their product kept growing in different ways such that the older models appear obsolete although they would work perfectly as a phone with a more limited set of capabilites.

EDIT: Oops, I forgot to mention their money sucking ecosystem that keeps the dough coming in! (your second half of the equation!)