I used to work for Nokia back when they still made phones. Replaceable batteries were very normal then. Phones were a bit thicker than today but not massively so. These days phones are actually thinner but much larger. I had a "are you happy to see me or is that a Nokia" type Nokia 9300. That was a brick. But it had a full hw keyboard. and flipped open it wasn't that much bigger than a modern smart phone.
You could argue that the trend towards more energy dense batteries and wireless charging could enable new interesting form factors. Recent phones have magnetic connectors for external wireless chargers/batteries that snap to the back. Most of bulk and weight of a phone is for accommodating batteries. You could make an argument that making a phone with replaceable batteries is easier than ever. Many cameras have a bulge for the camera. The negative space of the rest of the phone could easily hold a swappable battery. How critical are those 3mm really?
You could argue that the trend towards more energy dense batteries and wireless charging could enable new interesting form factors. Recent phones have magnetic connectors for external wireless chargers/batteries that snap to the back. Most of bulk and weight of a phone is for accommodating batteries. You could make an argument that making a phone with replaceable batteries is easier than ever. Many cameras have a bulge for the camera. The negative space of the rest of the phone could easily hold a swappable battery. How critical are those 3mm really?