But just because a seatbelt can't protect you from the flu doesn't mean it won't save your life in a car crash.
Equally you are more likely to be killed by a vending machine than a terrorist attack in the US so why did Trump want the Muslim ban.
Risk management and assessment comes in various ways and methods.
I'm not anti-trident by any means. I'm probably more pro than anti, but the argument you use is not necessarily sound.
I think there is certainly something here: we've got caught up in ideas about what is an attack on our society coming in a particular form and from a particular source and haven't been smart enough to see it evolve and come from a different direction.
Our reactions to these threats are often very behind the thinking of those carrying out those threats rather than being truly preventative. And even as we get better at being preventative, our enemies find newer ways to attack us anyway.
There is a lot to be said for the fact that we look at trident as a safety blanket that will protect us from threats. The reality is that others realise that using them isn't the best way to wage war anyway and they'd be much better using other methods.
I've seen it suggested that Russia's involvement in Syria, isn't about ISIS nor Assad at all. Its about creating refugees that destabilise the world allowing it to take advantage of the cracks that this produces throughout the world to expand its influence.
I'm yet to be able to effectively think of an answer that totally dispels that as an objective.