Why treaty limits could hold

What stops a navy from retaining old ships beyond treaty replacement limits when a suitable replacement is unavailable?

Cost and obsolescence matter, but the deeper restraint is reciprocal security: a navy that keeps extra ships invites competitors to do the same and can undermine the ratios it relies on.

Keeping older ships can preserve nominal tonnage, though it cannot deliver the hull form, machinery, and combat improvements of new construction. A navy may retain old vessels, and other powers can answer by keeping or building more of their own. Japan depended especially heavily on the agreed ratio because British and American industrial reserves could outpace it once competitive building resumed. Treaty compliance lasts only while each signatory believes the arrangement protects its security. Once that calculation changes, formal limits lose force.