Practically it could work out very well. The Big Three might voluntarily standardize on a battery formfactor and swapping bay, because it would put them ahead

Practically it could work out very well. The Big Three might voluntarily standardize on a battery formfactor and swapping bay, because it would put them ahead of Tesla. "Our patent is free, follow our lead Mr Musk." There could still be plenty of flexibility in shape, but it would likely be constrained. Like, the width/depth must be a fixed number of centimeters, and there must be a common connector and common mounting points. But the length is up to the manufacturer, programmed into an RFID tag for the swapping bay. (And you'd be able to use shorter batteries inside a larger vehicle.) Battery advances are the most trivial of all to incorporate. Already li-ion packs require dedicated charging circuitry. The exact cells used don't matter, a tenth-of-a-cent EEPROM is programmed with the charging curves of the cell chemistry. If anything battery tech will advance much faster with swapping. Without swapping you have to be very conservative. Advances can only be introduced with new models (so once a year) and are very costly to get wrong across a fleet. With swapping you could put a couple of prototypes into the pool every month, and swap them out immediately if the prototype underperforms.