It's not just hull life that is a factor. A submarine is an incredibly complex machine with a huge number of big moving parts. 3000 PSI hydraulics, 4500 PSI air systems thousands of valves for air water and hydraulics, hundred of hydraulic rams, miles and miles of piping, miles and miles of wiring. All of it subject to high stress on a routine basis. The boats I was on are pushed hard with an optempo that will wear out both machines and men in short order
Both of the boats I served in were getting old by the time I got to them.
The USS Haddo (SSN-604) had already been in active service only a few years less than I'd been alive. When I got there it was a tired boat ready for overhaul. I rode it through the yards at MINSY returning to the fleet in 1984. She was Decommed in 1991 with only 26 years active service. I had a chance to talk to a couple of guys that were on the decom crew, it was worn out.
https://en.wikipedia.org/wiki/USS_Haddo_(SSN-604)
The USS Flyin Fish (SSN-673) was a whopping 6 years newer. I got to it fairly fresh out of an overhaul and rode it on 2 major deployments. Even fresh out of overhaul *expletive deleted*it was constantly breaking. Mostly it was little stuff but there were a few sphincter tightening events. It was Decommed less than 4 years after I left. It too was just flat wore out after 24 years active service.
https://en.wikipedia.org/wiki/USS_Flying_Fish_(SSN-673)
There is only so many times you can compress a hull. And yes when you take one deep the walls do close in on you. I've seen the old string between the frames trick many times.
My father served on a WW2 Diesel-electric, an old Gato class fleet sub, the Cavalla, after he got back from Korea in 1953-4. The Cavalla was built about 1942, was credited with the sinking of one of the Japanese Carriers that attacked Pearl Harbor. When my father served aboard the Cavalla it had received a new sail, hydrophone array, and had been reclasified as a
"Hunter Killer." At one point during its WW2 service it had been taken deeper than its theoretical crush depth and had developed a leak.
It still had this leak when my father served aboard. This sub is now at a Naval Museum at Pelican Island in Galvaston Texas.
A book I own about WW2 Fleet Submarines recounts one submarine that had barely survived a Japanese depth charging. It barely made it to the surface. Upon cracking the hatch, the crew found the decking had been lost, and the pressure hull had been stoved in, "scalloping" the hull between the bulkheads.
That must have been a puckerfactor moment....