However, a drone would be useful in sub hunting and could carry a couple of missiles.
Potentially, yes. But the configuration for such would essentially be as expensive as replacing the S-3.
You would have a trade off in terms of interception speed (S-3) vs. loiter time (UAV). There likely is a strong case to be made for BOTH.
Not an either-or black and white decision. Which unfortunately the White House has not been good at.