Apple is capable of offering full backward compatibility and they should. There are a lot of fine apps on iOS and applications on MacOS that don't run with the new's iterations of the OS and nolonger have developer support. This is a flaw in Apple's corporate culture. They would sell more hardware if they offered full backward compatibility. A lot of older hardware is kept around and people don't do upgrades to the new hardware and new OS because apps they use won't run on the newer systems.
Actually, they sell a lot more Hardware this way. A couple friends of mine are still using iPhone 5. guess what, they're planning to upgrade because they don't have a choice if they want support.