Why a wartime fleet still needs replacement ships

Could the U.S. Navy rely on surviving older ships if Japan no longer posed a major late-war threat?

No; years of wartime use would leave many ships too worn, so new hulls would still be needed as replacements.

The scenario asks whether the United States could stop major new construction and rely on surviving battleships, carriers, cruisers, and destroyers. The answer points to accumulated wear rather than enemy numbers alone. Ships operating through years of wartime service become tired, maintenance-hungry, and progressively less dependable. New construction is therefore needed to expand the fleet and replace hulls whose useful readiness has been consumed by war.