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It's official: Smartphones will need to have replaceable batteries by 2027
(www.androidauthority.com)
This is a most excellent place for technology news and articles.
The main factor to consider in making an ultrathin phone in 2023 has nothing to do with the battery. It's the requirement for a certain level of build quality to be suitable for end consumers. At some point we just need to develop new materials, because we can't make it any more ultrathin without it also becoming ultrafragile using the materials available.
It hasn't really been a focus since we realised back around the iPhone 5 that making these sweeping compromises for thinness was yielding diminishing returns and causing other problems. Today that's still the thinnest mainline iPhone, only the SE and 12 Mini are thinner. 13 mini is thicker, and there is no 14 mini.
Ergonomics matter too. At this point going thinner is purely a marketing exercise rather than a practical improvement of any kind. If they were able to businesses would be making them so thin you can't hold them without risking a paper-cut so long as that allowed them to convince people that meant it was better than their current, designed for human hands, smartphone. Same thing with size. Personally I prefer a larger display and am willing to accept slightly worse ergonomics for it but even with more or less average sized hands I definitely find phones with 6 inch or under screens much more comfortable in the hand than the more typical sizes today and I know plenty of people with smaller than average hands (ie, half of the population) who really hate holding modern gigantic phones (and so often have held off on upgrading to a new model until I've steered them to something the same size as their old one.)