Why modern patrol boats cannot replace frigates

How would wartime small combatants compare with modern Royal Navy patrol boats?

Modern patrol boats have far better sensors, but their policing and presence mission is different; armed small craft would supplement rather than replace frigates and destroyers.

Technical comparison and mission comparison lead to different answers. A modern patrol vessel benefits from much better sensors, yet it is built mainly for presence and maritime-security work rather than a gun fight with a heavily armed Dog Boat. Small, fast combatants could support operations around chokepoints and allied waters. Such vessels would remain adjuncts to frigates and destroyers rather than a reason to cut them.