Janus was a prewar destroyer still performing demanding frontline work after receiving wartime modifications. Expecting a navy to replace whole generations of ships during a long conflict is historically unrealistic because warship construction takes too long. Aircraft changed faster in the Second World War because contemporary airframes were much quicker and cheaper to build than modern combat aircraft or warships. A future prolonged war is therefore more likely to produce new weapons and systems fitted to existing hulls than an entirely new fleet delivered in time to decide the conflict.
Notable Point · 42:37-43:55
Why long wars modernize existing warships
Long construction times favour fitting new weapons to existing hulls during a prolonged war.