not all aircraft have keypads
Good grief.
The cold hard facts are; The 2 on the flight deck at all times rule was mandated by the regulator in Australia as a direct result of a disturbed 'German Wings' FO deliberately flying an aircraft into terrain. However....the 2 on the flight deck rule does not prevent either Pilot destroying the aircraft and its occupants at any time.....if they so desire! So the rule does not achieve its objective other than it
may have an influence on where and how the job is done.
Nobody would object if the few operators of aircraft 'without keypads' develop procedures, such as the 2 on the flight deck, to mitigate the risk of Pilot incapacitation at an inappropriate time......but that does not mean there needs to be a blanket rule for all operators on all aircraft with more than 50 seats, when in the majority of cases the risk is alleviated by other means......such as a key pad.