Does the age of the frame matter that much? If the frame is taken back to bare metal, inspected and relifed before being reconstructed, where is the problem? If some systems are carried over and not modernised, I can see that being a problem, but I thought it was a ground-up rebuild with basically just the fuselage framework being carried over. Is that not the case? One of the things I see as frequent SLF that gives me a reasonable feel for the age and condition of the ac is the condition of the door area on entering. Old beater DC-9s and MD-88s have more coats of paint than a Navy ship, and the rubber seals are often in rag order. Lo and behold, the rest of the interior usually follows suit. Are the MRA4s being reworked to that extent?
I do see that the frame is perhaps too small in cross-section to begin with, but that's a different issue than if it's knackered or not. Never been in a Nimrod, but I did fly in a Dan-Dare Comet as a kid, and again about 10 years ago went on the Comet at Duxford. What struck me was how small it was.
Sorry for the noddy contribution, I know sod all about Nimrods, but the basic idea seems pretty simple.