Yeah, you have a good point.
Any AA (American Airlines) frequent flyers here on FR know if this is true? I have heard from AA flyers that if one of the 2 pilots steps out of the cockpit for the restroom, etc., a flight attendant steps into the cockpit & leaves a service cart behind to block the aisle while they are in there. Pilot comes back, knocks, gets observed by the FA thru the cockpit peep hole & gets let back in. This eliminates the 1 pilot by himself point of failure (heart attack, shenanigans, etc., etc.). Anyone see that procedure on any of their flights?
Up thread someone mentioned a combo key pad to get in normally from the outside, in addition to a deadbolt mechanism inside to intentionally isolate/secure the pilots.