Sky Marshalls = Safety (So far)
Well yes , you can say that Sky Marshalls have worked , and have not been used/abused by being Got At . I would add "So Far" .
Having a small community of elite/select individuals makes things more manageable , but once you have large numbers of them , all needing Rostering , Training , Checking , Administering etc it will become impossible to keep 'confidential' who they are and which flights they are assigned to.
And if the whole scenario does seem too far fetched ( which I agree it might) : first , you are not trying to stay a step ahead of fools and idiots, but , as we have seen , intelligent and resourceful people with ambition and vision (well , enough of them to matter) . Secondly ; we spend many happy hours in the Sim thinking about Emergrency Electrical Configuration , Double Hydraulic Failures and so on . Even though the chances of these actually happening are 10 to the minus quite a lot we don't call that Dreaming up Problems with the Aeroplane , we learn how to deal with it .
You cannot argue that if something unlikely hasn't happened yet , it won't . Risk assessment requires more analysis than that .
In passing , allowing Pilots to carry guns onto their Flight Decks re-introduces the risk of being Got At on their Days Off not only to they themselves but to all other pilots that are identified with them , that is , all the Pilots who no longer must go through an effective Security Screening .
Are we happy with that ??