The current designs allow for fitting more battery into a smaller space because the available space can be non-contiguous and can contain odd angles while a rep

The current designs allow for fitting more battery into a smaller space because the available space can be non-contiguous and can contain odd angles while a replaceable battery comes in whatever shape you want as long as the shape is rectangular. By not having accessible batteries the vendor can seal the phone much better and increase its water-resistance. Your older phones were larger, heavier, not water-resistant, and had one more set of user-accessible failure points.