you can't keep increasing battery capacity indefinitely either, batteries become more reactive as the electrode potential increases, so there's physical limits

you can't keep increasing battery capacity indefinitely either, batteries become more reactive as the electrode potential increases, so there's physical limits to what batteries can (safely) deliver too. Unfortunately a lot of the battery capacity gains of the last 15 years have resulted from loosening these tolerances for fire/explosion, I'm thinking about lithium-ion and lithium-polymer chemistry here. Lithium-iron-phosphate is an example of a safer chemistry, but it also has significantly lower energy density, it's probably still better than NiMH but not anywhere near as much as lithium-ion or polymer types. That means that if you are looking at an iso-safety sense, the gains have been much slower than "maximum potential even if you are ok with batteries exploding when overcharged/overloaded/punctured". We have just been ramping up our tolerance for batteries of the latter type.