Why not heavily up-gun River-class patrol ships?

Why not replace the River class's 30 mm gun with a close-in weapon system or otherwise up-gun the ships?

Because extra weapons make the ship costlier and more complex while encouraging it to be sent into threats meant for frigates or destroyers; modest sensors, electronic warfare, drones, and a 40 mm gun better fit the OPV mission.

Electronic warfare, drones, fast boats, and perhaps a common 40 mm gun would suit the River class better than Phalanx or a much larger weapon. Electronic surveillance also uses the value of a sovereign platform that can collect communications while moving through a region. These systems improve awareness and low-level self-protection while preserving crew, cost, and maintenance advantages. Heavy armament can create a false impression that an offshore patrol vessel can substitute for a frigate, encouraging politicians to deploy it into threats its hull, sensors, and magazine depth cannot survive. A River should handle maritime security and routine presence. When missiles or high-end combat become likely, the navy should send a Type 31, Type 26, or Type 45 suited to that threat tier.