"Actually, I can't remember the last time the 5s lasted me a whole day. It usually dies down around 4pm, if I don't use an external charger.
The problem is, there is only so much optimisation [sic] you can achieve before the laws of physics take over."
One has to be careful about extrapolating a personal experience as the norm. You're most probably getting less than a full day's worth because your 5s isn't set-up optimally. Stuff like open apps, Bluetooth, Localization, roaming, bad installs, etc. can drain a battery quickly. So can watching videos, checking email compulsively, and so on. While I think no one would complain if they could get several days off one charge, there is only so much energy you can squeeze from a thin little cell. So, if Apple can add a bunch of nice improvements and add an hour or two (realizing 10-hours or so on a charge), I think no one will complain...too much.