Maybe I'm cynical, but I think that's part of the Apple treadmill to keep people buying new devices. iOS 7 is written for the new hardware (as it should be), and not fully optimized for older devices because they want people to buy new ones.
I have a 5 year-old laptop that works perfectly for my purposes, but I'll probably have to buy a new one just so I can run Xcode 5, and the only reason I can't run Xcode 5 is because it won't run Mountain Lion / Mavericks. Even then, that's only because Apple won't optimize Mavericks for the GPU on my machine - the same laptop can run Windows 8.1 just fine.
I have a 5 year-old laptop that works perfectly for my purposes, but I'll probably have to buy a new one just so I can run Xcode 5, and the only reason I can't run Xcode 5 is because it won't run Mountain Lion / Mavericks. Even then, that's only because Apple won't optimize Mavericks for the GPU on my machine - the same laptop can run Windows 8.1 just fine.