Hell, tons of software-driven things don't allow you to downgrade, it's not just phones. And often for good reason, a software upgrade on a modern "thing" often contains firmware or patches for various chipsets in the device, not just some OS or app-level features and fixes. Orchestrating a downgrade, and ensuring it doesn't accidentally brick something, would be a major testing headache across the matrix of all potential FROM and TO versions of software that could be involved. All so that some super edge case user can be accommodated.