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.)

And it had completely different innards and battery capacities. Just grabbing that old battery and putting it in a new phone would seriously limit the runtime on a single charge. Which is kinda the point, I really hope we don’t trade replaceable batteries for the need to recharge twice a day or switch batteries to even make it the whole day. Or have a noticeable bulkier phone that won’t fit as comfortably in my pocket. Or that it may not survive the rain shower I got surprised by because they skimped on the water proofing.