Given a fleet of submarines is $50B and the cost to fully instrument the entire ocean with sensors is $1B, is the price of a submarine worth it anymore?

Sensors don't attack. Sensors don't even defend themselves. They are one piece of the "kill chain", but they aren't the "kinetic" piece.

Do you necessarily need a sub to kill another sub once it's detected?

Surface navy and aviation could do the job.

We don't have adequate wartime data to know for sure one way or the other. Historically, there has only been one underwater combat engagement between submarines [1] openly acknowledged (there might be classified engagements). However, even generously accounting for all submarine lost at sea accident reports as hostile engagements doesn't yield sufficient numbers of engagements to know what would happen with today's subs between each other in open warfare.

Let's hope it stays that way indefinitely.

[1] https://nationalinterest.org/blog/the-buzz/the-true-story-th...

What do magic realists say?

[1] https://jamestown.org/program/a-smarter-battlefield-pla-conc...

The target doesn't have to be a sub. The sensors could detect a surface vessel, and the sub could be the way of attacking it.

A bomber can do it cheaper and faster.

Considering how many peoples lives were disrupted over that it is just sickening. What a bunch of bs that whole situation is/was/continues to be.