>It seems unlikely that they produced generations of submarines that were less capable than their older ones.

i wouldn't be sure. There seems to be no military advantage to deeper so why spend the money. A sub needs to hide, but it can't do any other job when too deep. sinking ships can only be done when near the surface. If the sub can get under a couple thermo layers that is good enough, any deeper is more a party trick than useful.

i'm not in the navy but that is how I read the unclassified information I have access to.

> sinking ships can only be done when near the surface.

Isn't this itself a huge assumption?

Sinking ships via upward facing torpedoes would be a huge tactical advantage at first glance. Less time to detection and deploy countermeasures or evasive maneuvers.

Perhaps, I am wrong in assuming they cannot be fired below a certain depth?

just watched: https://www.youtube.com/watch?v=V-mzZXwCn68&t=2s

while these torpedoes are going in the other direction seems like the technology for surviving, functioning and navigating depths definitely exist.