It's only just been mentioned after 8 pages, but the idea of the FA in the cockpit is simply to unlock the door and let the other pilot back in, not to physically prevent the remaining pilot crash the plane. So the FA only needs to know how to operate the lock, and the door handle.
I am amused how many of you are kicking off because you feel you are no longer trusted, but then you also say that you don't want some "unknown FA" or even "queen", (charming - how enlightened), in the cockpit ! Double standards and prejudice? Do you feel the same about a pilot you haven't flown with before?
The point about pilots going through security and yoghurts etc., is to prevent a pilot smuggling something on board which is designed to explode or cause damage after they have got off.
I don't like security or the new FA ruling either - it is more bother and complication for me and my working day/night, but I can understand the intent.
The real answer that has been suggested by myself and others is to keep the cockpit door as it is but move it rearwards so the forward toilet becomes part of the flight deck. A pilot will only be in the loo during the cruise, and so they would have time to come out and take control if the PF started doing something weird.