Type 26 moves the Royal Navy to a supported 5-inch gun

Why fit a 5-inch gun to Type 26 instead of retaining the Royal Navy's 4.5-inch weapon?

Because sole support of the declining 4.5-inch system was becoming expensive, while the widely used 5-inch gun offers better commonality and sustainment.

The Royal Navy had effectively stopped building new 4.5-inch guns. Continuing alone would make Britain solely responsible for the ageing weapon's supply chain, spares, ammunition, and technical support. Moving Type 26 to the 5-inch system joins a calibre used by other navies and shares more of that burden. A mature weapon can become expensive even when it still works because lifecycle logistics matter alongside ballistic performance.