The only threat they pose is to some other third-rate navy or shipping vessels that are pretty much running blind. If push comes to shove they will become underwater reefs very quickly.
You might be a bit optimistic. Even old subs can be deadly; just like an elderly cobra, it's venom still works.
Then again I might not be, having spent quite a bit of time on one during my stint in the Navy. We played lots of war games with diesel boats, diesel boats a hell of a lot better than the ones we are talking about here. And we ate their lunch every day of the week, even in an old 637 class boat that was far from top of the line, even 20 years ago. I know somewhat of what I am talking about you see, having been there, done that.
And yet, in spite of this technology, the Norks sank a state-of-the-art South Korean naval ship in 2010. see:
http://www.bbc.com/news/10129703A report on the incident stated:
"The evidence points overwhelmingly to the conclusion that the torpedo was fired by a North Korean submarine.
"There is no other plausible explanation."
Something must be worrisome in the South's naval bureaucracy.
