The value approach would be to make wireless charging standard at $149, instead of the stacking approach where it is simply added to the bottom of the stack.
The XR being sold as a cheaper alternative to the 11 is also a form of price stacking, designed to entice consumers with a lower point of entry, after which they see the missing features and end up looking further down the stack to the 11. There is clearly a lot to justify that $100 difference, so for the price stacking strategy to work it was decided to not lower the price of the older generation iPhones any further than $100, otherwise the top of the stack has too much value and fewer consumers will be looking further down.