Black Swan fell between OPV and frigate requirements

Why was the Black Swan sloop concept not used as a River-class OPV successor?

It was too expensive for patrol duties and not capable enough for frigate or destroyer duties.

The 2012 Black Swan concept might have filled the gap above a River-class offshore patrol vessel, but its position in the force structure was unstable. It was judged too expensive and complicated for OPV duties while lacking the combat power expected of a frigate or destroyer. Even Type 31 was likely to carry more firepower, making Black Swan a useful warning about designs that do not fit a funded role.