I'm sure we will get recomendations and these must be acted on universally by the industry to ensure that nobody can be left alone in a locked flight deck door...
Trouble is you are now designing a system to cater for a suicidal pilot... which rather admits there could be such individuals out there.
The regulation and industry relies on pilots to be the solution, not the problem. Once you swap that, there are plenty of other problems unearthed.
Remember, we have single pilot Ops up to a certain size, and serious studies being made into single pilot airliner Ops, especially in cruise / Long Haul.