Large ballistic missile submarine fleets and a military posture that makes an attempted first strike likely are the main reasons to maintain such a net.
China has 7 active ballistic missile submarines. The US has 35 active nuclear attack submarines.
It's probably cheaper to run a less thorough net and then task subs to shadow any contacts made.
>> Large ballistic missile submarine fleets and a military posture that makes an attempted first strike likely
It was in the past. Today such things can be used for all sorts of intel tasks. Sosus is real time info about positions/speeds of all ships, surface or not. Satellites cannot do that. I imagine that the US is also using sound to monitor Chinese building activities in the south china sea. There is also, probably, some economic intel to be gained by monitoring shipping/fishing fleets, again, in a way not easily replicated from space.
China has 7 active ballistic missile submarines. The US has 35 active nuclear attack submarines.
It's probably cheaper to run a less thorough net and then task subs to shadow any contacts made.