My younger brother worked in those babies in the 1970s. They were already full of new wire with the original wire harnesses still in place. Pulling the original ones out would be too expensive. Not only that, there are many "stop drills" and pop-riveted plates to repair air frame cracks. This makes the craft a great deal heavier than it is supposed to be.
For the life of me, why don't they build new ones using upgraded designs to replace the dangerously aged hulks? It is a wonderful design that should be replicated in newly built aircraft.