I suspect that a good part of Samsung's slowing sales is consumers that are tired of spending more money all of the time to do the same thing. I've got a Galaxy SII. It does everything that I need it to do. It's paid for. I don't foresee any needs that a newer phone would fulfill, so short of a broken phone or a paradigm shift I don't see a need to shell out several hundred dollars to have essentially the same functionality.
THere's a lot that an S4 or S5 will do that your S2 cannot - though you may not appreciate it, including things like Bluetooth4 or a larger battery, better camera or support for more innovative features on the latest Android (or TouchWiz) release.
Personally I have an iPhone and I upgrade every several years with my wife leapfrogging so one of us has the latest phone. Apple has perfected the upgrade treadmill - you're compelled but not forced into upgrading, and they make the features on each new generation worth the upgrade.