This makes me wonder, if you can replace one, can't you add one on top of the old one? Or two or three? There goes the range anxiety

Not yet, but I kinda wonder if it will happen. At least in certain situations. For example, the F150 Lightning -- body-on-frame truck where the battery sits between the rails rather than being tightly integrated with a custom unibody design. As batteries improve, I could see people retrofitting newer batteries with more capacity.