The corridor beyond De Akkers station was indeed intentionally left unbuilt, this to allow an extension to Hellevoetsluis and Brielle. Especially Hellevoetsluis has a quite large commuter flow to Rotterdam (the town expanded rapidly in the 70s and 80s to house commuters working in Rotterdam and the harbour), with buses towards Spijkenisse running every 5 minutes during rush hour. The same applies to Brielle, which has up to 10 buses an hour towards Spijkenisse.
So there's actually a quite significant demand for public transport there, although currently there are no plans to extend the metro line. Instead a bus lane with grade separated intersections was built, allowing to also serve the intermediate villages frequently.