Roadster,
I’m no expert (I have worked ships through refit) but I was on the Invince for her last commission and I have spent my whole working life in the RN.
The Invince (for example) was rotting structurally and her systems were far from functional. I can remember the joys of the Norwegian trip with no accommodation heating (lovely!). The whole of the last commission was carried out with a refrigeration container in the aft hangar because the ships unit was knackered beyond repair. The Ark (which was standby at the time) was robbed blind to keep the Invince running, although as you say, the engines are easy to change, I know the stokers were getting stuff from the Ark flown out to repair the engine ducting. Anyone who ever served on the Invince will be able to tell you tales of her dodgey shaft, which reared it’s expensive head every now and then.
The use of “exotic” materials in the construction of ships would be a non starter, the specialist skills required to work some of these materials is not easy to find on an industrial scale. Add to this the cost of these non-corrosive materials, you could end up increasing the build costs beyond what would be economic (they would say, “why build to last 50-60 years when 25-30 will do?”).
Don’t forget that any bits required for these ships now are going to be bespoke construction, I would imagine the support contracts are long gone and any “one off” lump of metal or component costs big time. (That tends to be the case with any ship class that’s nearing it’s sell by date.)
It’s relatively easy to keep a museum ship afloat, you just keep pouring quick setting cement into the bilge (Ah memories!) but if that ship has to move and fight you are in a whole new ball game.
I suppose in the end it’s a simple calculation based on cost to build/buy for 25-30 years use, against refit for one more commission and hope nothing big breaks. (I don’t even know whether seaworthy certification comes into that calculation)
Cheers