There's a middle ground here though. The iPhone 4 is legitimately pretty shitty on iOS7, but the 4S runs the newest OS just fine. I've tested some pretty demanding apps on 4S/iOS7 and the thing barely breaks a sweat.

And that's a 2 year-old phone. I don't think it's unreasonable to expect at least one major update out of new hardware, even low-end.