Currently, are F/A's prohibited from entering the flight deck when one pilot is out? Not at my airline & I haven't seen evidence of it at other airlines either. So all this concern about a F/A in the flight deck with just one pilot seems a little misplaced, as there has been nothing preventing it for some time now.
It's a valid point by itself, but there is no disputing you are adding another human to the mix. A human which is as at least as likely to become the perpetrator as the pilot. The point being that with the statistics involved, no matter how minute the odds, it doesn't support a safer enviornment unless you assume the perpetrator is on the outside. (Which is what SHOULD be happening)
If you assume the perp will be a crew member, you just increased the odds of having have happen exactly what the misplaced fears are about. So this "fix" as applied is inappropriate to the recent occurrence which spawned it.