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.

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.)