The C130Ks were reboxed and planked in the mid/late 70s because the old wings were crap. There were some wing swaps around 2002 time because some of the Mk1s were End of Life. Redoing the wings in the 70s/80s was clearly worth as we'd only had the things about 10 years and they soldiered on for another 20 or 30. Rewinging of a couple of aircraft in the early 00s was a sticking plaster to mask general procurement failings elsewhere. Was that worth it? 50/50 I'd say.
The TriStars are old, complex aircraft. The Herc, whilst also old, is mainly just a few bits of metal bodged together with some engines bolted on. The TriStars are tired and knackered and far too complex to keep running from the Defence budget. Nowadays you need the off balance-sheet magic and sleight of hand of PFI if you want to fund big projects.
One would have to be more than a little deluded to think that keeping the TriStar running would've magically given the RAF £13 Billion to do with as they pleased. The Harrier was sacrificed partly to save money but mainly to stiff the FAA. The Nimrod MR4 would've consumed all of that imaginary £13 Billion and still not be right.
Yes the USAF run older aircraft however they also run to very different levels of risk acceptance. I'll be interested to see what was behind the midair break up of the KC135 recently....
The modern RAF is more risk averse than a primary school road safety lesson; there is no way on earth anyone would want to sign off on yet another bodged-up update of a knackered old airframe nowadays.