People generally want their gadgets to be as lightweight as possible, cheap as possible, last as long as possible, and be reliable. There's tradeoffs in balancing those. eg: overbuilding the battery to make the device run longer in the face of degradation adds weight, size, and cost. Somebody has to make a call on where the balance should be.
What nobody really talks about in the context of device longevity is wear levels in the onboard flash. A battery replacement or three doesn't extend that clock. It's pretty good but it doesn't last forever. This is more of an issue on devices with smaller amounts of flash storage with a lot more storage churn.