We (ie the government, airlines, and people who do a bit of research into these things; not sure if the Public At Large do or not) know perfectly well that well-planned terrorist operations can breach security, and will be able to for as long as there are low-wage civilian jobs in airports.

We also know that there will always be low wage civilian jobs in airports; otherwise the cost of flying would be impossibly high and everyone would be far worse off (unless you’re a Green who believes flying is next only to genocide in the scale of Very Bad Things).

We also know that real life is not like 24 or a Harrison Ford film. If there’s a gun battle 10,000 metres up between terrorists and armed policemen with 300 civilian bystanders, the outcome is rarely going to just be Bad Guys Dead, Good Guys Live.

We also know that military planes can easily shoot down hijacked airliners. This is why worries about another literal 9/11 disaster are completely misfounded: no government will ever mess up its air defences quite as radically as the US did that day. That’s probably unfair to the people involved: given that as far as anyone knew the US faced no incoming airbound threats, they were totally confused by what happened. But that won’t be the case if there’s a next time.

So the only thing sky marshals can help ensure is that hijacked airliners crash into unimportant targets – and this can be achieved by air defence already. What a brilliant use of resources…