Sadly they don’t do it because it would allow downgrading. You can’t sign iOS 12.5.7 for every device originally compatible with iOS 12 because that would mean that everybody can downgrade. They can’t support older iOS versions forever, so there would be a time where iOS 12 would have no more security updates.
But regardless of that, they don’t do it because, like I said, it would allow downgrading to iOS 12 from iOS 17, and they don’t want that.
So they support iOS 12 with security updates for now... for those device that aren’t compatible with iOS 13 onwards, which means that a Xʀ user on iOS 12.0 can’t install it.
Regardless of all of this, your point is the most important: Apple shouldn’t force a choice between security and battery life/ performance. I’ll choose the latter every time. So you either provide both or I’ll make do without it.
