Apple should allow downgrading the old iPhone models to older iOS versions.
While I agree with you, it will (probably) never happen. Apple has too much self-interest involved to ever allow this.
First, it fights jailbreaking - which they hate because it allows you to do things outside of their control.
Second, it creates (for them) a support nightmare. Having most users on the same version of iOS (or close to it) allows for smoother heterogenous support. Apple doesn't have to troubleshoot issues on a wide range of devices and versions of iOS. They can focus on supporting a very narrow range.
Third, Apple is (they say) all about security. Allowing users to downgrade means less security (to Apple).
Fourth, it looks good for Apple to say "X amount of users are using iOS version X".
Fifth, they are a hardware company. Selling you a new device with a newer version of iOS is preferable to them then allowing you to continue using an older device on an older version of iOS.
Some have suggested that Apple could allow downgrading by providing a warning, thus making any issues or problems solely the user's responsibility - no support. Again, Apple won't do that because people don't consider things like that as any sort of legal barrier towards suing Apple. Or in seeking support from Apple anyway.
Apple isn't into allowing the customer full control of their device. We don't even have ownership of our own copy of iOS. It's not how they operate. They want the experience they want to give you to happen according to their rules. That can't happen if you're not within their control.