you couldn’t even replace two AA batteries with a single Li-Ion cell.
That can actually work. A single 14500 Li-Ion along with a dummy battery can work in many devices, as they have regulators for the voltage in the device (it might blow up other devices and won’t give any energy advantages). That said, there is very rarely any use for this these days, as you can buy 1.5V AA Li-Ion batteries with the voltage regulator build in, that will work in all devices. More expensive than NiMH however, so only really good when the device absolutely needs the 1.5V and isn’t happy with the 1.2V of NiMH.