You guys know lots more about this than me but I would like to give you something else to think about.
When Apple announce a new model, it’s full with new features, bigger this faster that etc.
To accommodate this a new iOS is developed shortly before (Beta) l believe and it’s tested by people on their present iPhone models for faults and stuff.
When Apple are satisfied it’s released and the World gets to use it. Some features are only available on the new iPhone model. However the Security Updates are available to all.
My NOBODY theory is simply that the existing is iPhone model is finding it hard work coping with the new the
iOS especially with a battery that could be more than a
year old. And simply needs changing.
A great solution from my point of view for my iPhone 5s for example were none of the iOS 11 shininess applies would be to offer the Security Only Updates for iOS 11