That doesn't make my point moot. Allowing downgrades would increase the amount of time that they would have to support older versions and increase fragmentation for apps to deal with. Assuming that there was a significant enough demand for downgrades to make it worthwhile to allow them.
How would this be any different then a device that could not support later versions and is still covered by warranty? What you fail to realize is that it makes no difference. What fragmentation do you speak of? It's very simple. If an application requires a framework or library that the current version does not support, an error is displayed indicating the user must update. They can then choose to update to continue using the latest versions. Nothing has to be done or changed - it's always been this way.
Assuming that there was a significant enough demand for downgrades to make it worthwhile to allow them.
Obviously you don't understand how the signing process works. Apple has significantly gone out of their way to disallow firmware downgrades. There's nothing to implement for the reverse - it is literally a flick of a switch for them to sign older versions again.