Are you joking? I get the belief that tech should last a long time, and I typically get closer to your ten years out of a high end desktop. However, cell phones, while expensive, are a luxury item, and therefore treated as semi disposable after the typical contract ends. I've got about 3 years out of my 4S, and it is definitely getting buggy and slow even without updating to iOS8.
Apple has no intention of supporting it (4S) any longer than it supports the A5 chip with upgrades. Sure, they are still selling A5s in the iPad mini, but it won't make it past next year. I suspect the year after that, the iOS won't support it either. Apple (and all the others) don't make the huge profits from you sitting on old hardware.