Why do new iterations of Apple hardware keep battery life constant while single-mindedly hacking away at thickness? The ATP hosts grappled with the question in their most recent episode.
To paraphrase: if a product keeps improving on the basis of performance, it will eventually over-serve its market leaving room for a cheaper/faster/simpler1 alternative to sneak in and disrupt it. But experience can never be over-served. A word processor can have too many features, but we'll never say "That UI is too effortless," or, "That interaction is too delightful."
Apple is very cognizant of this. Battery life is a performance-based metric whereas thinness is experiential. That is, there is some (huge?) amount of battery life past which we simply won't care anymore. But we'll never say of our devices, "This is just too thin."
By pushing experience and holding the line at performance, Apple's protecting itself from disruption.
1. Note all these metrics are about doing less. It's important for the incumbent to see the competetor as being inferior.↩