Why navies build apparently obsolete ships

Technological change looks linear in hindsight, but naval planners have to choose while costs, reliability, and operational value remain uncertain. A ship is a system of systems, so the material or propulsion method that looks newest is not automatically best for every role. Glass bottles, plastic containers, and aluminium cans provide a simple example: technologies can coexist or return when priorities change. Older and newer naval technologies can likewise remain in service together instead of forming a clean sequence of replacements.