andyy, in so far as those warships built on aircraft style production systems you are right. I would guess that they would not necessarily have to be replaced like-with-like whereas you certainly could not afford to buy in a singleton MPA type.
A ship is a ship and is manned continuously by its crew whereas an aircraft is manned by a succession of crews so commonality is essential; that is the difference I was alluding to. They need to ensure a couple of MR2s in storage to provide essential resilience.
PN