Why the surviving battlecruisers were near retirement

Could Renown have remained in service much longer without postwar austerity?

Probably not for long; prewar plans already treated Renown, Repulse, and Hood as ships approaching scheduled retirement.

The surviving battlecruisers were not expected to serve indefinitely even after costly modernization. Repulse was due to retire in 1939 at about twenty-three years old, Renown in 1940 at about twenty-four, and Hood in 1941 at roughly twenty-one. War and treaty-era delays overturned that replacement schedule. The dates place their Second World War service in the context of a fleet forced to retain ships that peacetime planning had expected to replace.