And do you really think that ONE sky marshal can adequately monitor and defeat half a dozen terrorists, spread through the cabin- all quite willing to die? Don't think so.
First, there will never be ONE sky marshall on a plane. They work in teams.
I strongly suspect that the sky marshalls goal will be to get to the front of the aircraft and defend the cockpit while the pilots land the aircraft. Attacks towards the sky marshalls will thus have to come up the aisles towards the marshalls. So, can 2 armed sky marshalls, positioned at the front of the aircraft, defeat 6 terrorists armed with contact weapons coming towards them up the aisles or over the seats? I suspect so. Quite easily in fact.
The only way for the terrorists to overpower the marshalls is to close with them in force. If the terrorists spread themselves out in the cabin, then the terrorists cannot concentrate their forces.
Regarding the chances of sky marshalls being disarmed, yes about 20% of police officers in the US who are murdered are murdered with their own guns. As we've discussed previously, marksmanship training for the average US police officer is poor. Similarly, the average US police officer is also poorly trained in handgun retention. For example, many US police officers will get a couple days of handgun retention training during their police academy but never have any retraining nor any qualification.
It is my understanding that the US sky marshalls have excellent marksmanship training. I'm sure they also have excellent training in handgun retention and close quarters combat. Trying to take a gun away from one of these guys would be likely be one step short of suicide.
There are plenty of handgun retention systems. I have been trained in the Lindell and Insights systems, but there are plenty of others. I'm sure the US sky marshalls are trained to a much higher standard than I have been.