It isn't either/or. Satellites and undersea cables serve different use cases. Cables are great for high bandwidth communications between fixed points but they aren't very useful to mobile military forces and they can't be used for anything beyond communications. We don't have enough ships and patrol aircraft to realistically defend undersea cables outside the littorals.

Satellites can serve multiple purposes including communications, navigation, overhead imagery, signals intelligence, weather, etc. They are also vulnerable, but it's possible to launch replacements faster than repairing damaged cables.