Well unless you work in the relevant sector, you can’t say there’s no black market for anything just because you don’t see it. That is why it is called black
On the face of it, keys sounds a good solution, but in short Crew iPads go missing all the time, not good for keys. Then the pilot on the cockpit still needs a way of refusing access, key or not, in case the person with the key has bad intentions. The more likely solution is allowing FD access to the toilet without the need to go “into the cabin”. Some sort of double “airlock” door system, I believe the FAA are looking into it.
https://onemileatatime.com/news/airp...-deck-barrier/