Surely it's not mandatory though?
Why does it need a service? If you're happy with it as it was, tell them to leave it be unless they can provide you with their reasoning.
They may have checked its amplitude or suchlike and that may be low, but they need to tell you why!
They said it's at the age where it needs one. They're also replacing the crown, bezel and stem. I'm going to go ahead with it anyway as my lodger had his done and it came back looking like new.