the problem with our 20m cannons that i wish them to be replaced is they are manned and stabilized and i believe such gun have no place on a ship I even don't like them on fixed installation but at least on ground you can hit some target with them if you are trained on ships even Silvester Stallone with the help of a platoon of Hollywood special effect specialist and receiving direction from George Lucas cant hit anything with them
Manned light guns and cannons are necessary even today. Remember RWS are good for all weather and most of them are capable of firing low moving aerial targets.
But reasons to not have all guns aboard RWS are;
1º.- They are expensive. Even 12,7mm RWS can cost some hundreds thousands of dollars. It is not so simple to buy some servo motors and cameras and fit them into a 0,50 inch automatic gun. You must treat all pieces to make them seaworthy and long lasting, and that requires a lot of $$$$.
2º.- Logistics are challenging. They are prone to have malfunctions on their IR/EO devices and failures in their servo engines because of the salty environments.
3º.- Useless in case of combat damage. A modern warship have a CIC and sensors and specialized crew. If some combat damage is received all guns will be shotdown. Fire can cut electricity. A naval mine can take down or damage connections and plugs that would feed electric RWS. Even a light missile (MBDA brimstone have been designed for anti ship missions too) can shake and pull up the RWS from their rings.
4º.- You need specialized crew for maintenance and... operate them.
A manual gun can be manned by any member of the crew. It can be repaired fast and easily in the ship workshops. It is independent from electricity grid and can be used even after extensive combat damage.
You can see how some MK 38 Mod2 from the US Navy can be used in RWS mode and manned. That means that even US Navy doesn´t put all the egg in the same basket and trust in manual mode gun combat use.