How do you know what job they may have to do in the near future?
We've never had to use Polaris or Trident in the past either, but we still keep them as a deterrent. We have announced to the world, that we won't have them available for 3 years. Furthermore, 1) if they not of any use, why did we have them in the first place? 2) If they are not really required, why are we replacing them in 2020? 3) Why do other Navies have similar missiles then?
In a potential conflict, an enemy ship could fire on our ships within range of their missiles and our ships would have no means of return fire until within Gunnery range. If they lasted that long!
The naivety on display is astounding.