Draining a battery in a device is not the draining that kills it. Device limits the drain by itself. After that, the battery simply shuts down and refuses to provide power to the device until charged.

That's why old phones that shut down could be powered again for a while whereas Li can no longer do that.

The reason the device saves a few percent is so that it can still wake up on alarm and handle charging if battery is "dead".

If emptied to zero (battery cutoff at approx 7%) and the left discharged, then self-discharge will bring it below the threshold and destroy the internals.

You can't kill a battery by using until dead. You can kill it if left dead a long time, especially in a pocket, in heat.