Then they should offer models that are cheaper to manufacture, like the 5c. I'm surprised not to see a successor to the 5c. Maybe they are worried it would be too popular and gouge sales of flagship models negating some of their premium branding.
I'm currently on a 6s and see no update path for myself, I would happily sacrifice faceID and some of the higher-end phone materials for a modern paired down iPhone. I have about as much confidence in this appearing though as them fixing the state of the keyboards on the MBP's.
The 5C was a sales flop. In Australia, the 5S was outselling the 5C by eleven times [1]. Even in China, people were buying the more expensive iPhone rather than the plastic 5C one [2].
[1] https://www.smh.com.au/technology/australian-telcos-struggle...
[2] https://techcrunch.com/2014/03/24/apples-iphone-5c-sales-sto...