I think Apple would have to have a highly backwards compatible and modular iOS for that to even make sense. I have an old iPhone 4 running iOS 6, and I don't think it passes for a useable device. Apple could make the latest iOS backwards compatible, and find ways to reduce computing requirements by omitting functional or visual features, but that would completely counter the current artificial obsolescence tactics. It's unfair to offload those to a developing market, until they decide to support a greater fragmentation of the platform.