I don't know why we don't make them, upgrade them with modern technology, use straight explosive torpedoes, and use them to patrol our coasts. With the deck gun, it would be heck on drug shipping.
Use them to be ready to take out offshore oil rigs. Patrol the straits around Somalia, and sink pirates. You could make tons of them, cheaper than these huge tubular behemoths we currently use. You could arm them with a supply of tactical nuclear torpedoes that you could stand off and shoot into an enemy's harbor. Surprise!
With the proper configuration, you could make some into troop carriers, and have fully equipped landing craft mounted on the outside of the sub. Just climb out of the hatch, and drop the boat down into the water. A boat that already is packed with ammo and other equipment in waterproof packaging.
Well? Why not? A fore and aft 5" deck gun, machine gun mounts for .50 cal machine guns, and a mission to harass shipping and coastal facilities, and enough ground-pounders to be able to either board and seize ships, or sneak ashore and wreak quick havoc and then retreat back to the sea.
I'd have them travel in pods, and if one got attacked, the others would circle the attacker like hyenas and torpedo the piss out of him.
It would be no problem to provide them with modern non-cavitating propellers, and I see no reason a nuclear power plant to propel the boat, and de-salinate water and keep fresh air being produced. They could shadow our carrier fleets in Wolf Packs, maybe several packs, and free up our big hunter killer's and Polaris nuke subs to go do their job, hunting enemy subs, and waiting offshore to begin taking out enemy cities as needed.
I think I'm onto something here...