If you restore from a months old backup, then you’ve lost months worth of data. That’s not a great experience if you can’t backup before downgrading, and super hard to explain to the user why. What you may or may not have “heard” is not a good sample size for decision making.
Just because your phone syncs to the cloud doesn’t mean the way that your data is stored is somehow immutable. It’s extremely common to have various “versions” of people’s data (schemas, most likely) that get migrated once all of a users devices have gone into a new version. This was public in a recent version of notes which said new features would only become available once all of one’s Macs were on High Sierra and all iOS devices were on 11. Restoring on a recent version post-migration IS different than a downgrade.
I believe companies should support old hardware, and Apple clearly prioritizes this as well. Moving software forward is important for the whole ecosystem. As I’ve outlined, it is far more than 0 effort to support downgrades.