Hold on, I need to dust this old joke (“…pfheeet”): https://forums.puri.sm/uploads/default/original/2X/1/14296acc741cf94dc20ae2d948a6226712a8d28e.png (from: Size comparison to other phones - #41 by JR-Fi)
Using AAAs will drop potential max mAh and will make sure the device would be at least as thick as L5. But, on the other hand, current rechargeable batteries have 950mAh (as new, at least), in theory giving 3800mAh. So, 15-20% loss, depending on how good block batteries become. That’s relatively limited loss, if power consumption drops too, since potential for gains there is comparatively similar. So, no gains in use time, just more generic power source.
But let’s take it further. The space for battery would only need to be a millimeter or two wider to provide space for both battery options: a device specific block battery and for an alternative removable frame to use 4 AAAs. It wouldn’t be a big change to current structure. The question is, how different the next one (if there is a next one) would be by size and internal structure design decisions.