Why Boeing didn't employ two discharge limits - one for ground operations and one for emergencies in the air - is due to it adding complexity to the fault tree and the necessity to take into account the possibility that the ground discharge limit activates during an emergency, denying the flight crew power and possibly directly leading to an accident.
Why not have a dedicated battery for ground ops?
A simple big fat mechanical switch between the flight battery and the ground battery would not take much of a redesign?
Captain Macgyver could use the ground battery in the event of an inflight emergency.