Nothing’s stopping people from just playing on their old Gameboy so if the writer wants longer battery life, he can just do that.
Everytime I pull out my old Gameboy the screen with no backlight and the weight makes me quickly remember why I gladly traded battery life for power when I bought an anbernic handheld instead.
The Gameboy micro was as the name implies very tiny, so it didn’t fit a big battery. Backlight is also a very notorious battery drain, so combined with the small battery it’s not that surprising.