Why a mature heavy cruiser still needs successors

If Britain reached the practical limit of heavy-cruiser design by the late 1930s, what would justify building later classes?

A limit applies to the technology of its moment; radar and other new systems change requirements and force the design to evolve even if hull size stops growing.

One optimized generation does not permanently end development. Designers may reach a practical size limit with the machinery, armor, and weapons currently available, but later radar and other equipment alter the balance. A successor can therefore be necessary to integrate new systems effectively even when displacement remains similar. Physical maturity slows one form of growth without freezing the class in time.