The days of the big gun battleship are over. Hell, the ships in todays navy don't even have the structural ability to mount more than a 5 incher.
The role of navy gunfire support is fulfilled quite well by CAS aircraft.
It is a matter of timing and weather, my friend.
You can have the best air force in the world, but they can't keep the planes flying 24 hours a day thru bad weather from an aircraft carrier.
You never know when you will need CAS or where they are going to be in relation to the fight when called upon or even if they are going to be in the air at that time.
Naval Gun fire and artillery are able to fly thru any weather an aircraft can launch in.
And while the rail gun will be great... it won't be great until they are actually in use.
That is no where close to being right around the corner....
As of the last I heard, at 80 nm... the ERGM only delivers an 8 pound warhead....
The Marines would be better off adding a few more grenades each to their load....