Inquisitor:
What do you mean, 'they built them (the K) tougher in them days'? 'They', otherwise known as Lockheed-Martin, are the same company (albeit merged) that built the J. The airframe is the same in both types, it's what goes in them that's different. Or maybe LM use thinner metal nowadays, or only one spar where 2 used to do? Even if the K was 'built tougher', the 40 years of FI used on the K would surely even things up?
Granted, rather like an old banger of a car, the K might soldier on with a couple of crocodile clips & a bit of wire 'fixing' some systems, but if 'significant airframe damage' can be speed taped over & the K flown, why couldn't the same repair be done to an identical airframe? And are you suggesting that a K with 'prop & undercarriage issues' would have kept flying where a J couldn't? Methinks you are extrapolating one specific case too far.
Once again my personal experience here, of flying both types, is that the J beats the K handsdown for reliability, & so even given that your premise of the K being able to soldier on with more faults were true, it would have to, to be able to fly as often!