The battery in your current phone is not that much bigger than the replaceable cells of yore.

The big difference is simply that battery chemistry has improved and new cells have more energy per volume.

There is no technical reason at all that we can't have replaceable cells with the capacity of current internal batteries. Unless you count waterproofing, which I've always viewed as nothing more than an excuse for planned obsolescence. There are definitely ways they could engineer water-resistant phones with replaceable cells.