Check 'Janes' and you will see a whole host of nations that operate both nuc and DE subs! Whilst DE subs are more likely to have a mission objective in the littoral, operatin in that environment poses many problems. The Nimrod is as good a platform as any for operations in that area, indeed it is practiced regularly.
Besides, Ice Station Kilo seems awful busy if the threat has gone completely, perhaps the Nimrod doesn't just chase subs!!!??? Not saying any more on here but check the role of the aircraft!
As for the downer, it's all due to the project overrun and overspend, mainly through BWoS and bad management by the DPA (IMHO). The MRA 4 will be a good platform if it ever gets the proper backing, the project is only 8 years old and it's about to start flying- not bad really.