I don’t know much about nuclear plants either but I do know a bit about wires and I think you are correct. It’s very unlikely that one of these cables just snaps, look how they twisted the wires in that ‘arm’ so it can move and bend forming a nice elbow. So if it’s corrosion, temperature, mechanical shock… it could affect other cables as well making advisable to replace the whole bunch altogether.

That said I’ve seen a very similar looking wiring, though very different machine, an electronic organ from the 60s. Among other issues there were some dead keys on one of the keyboards and the problem was the wires. I just ran a new bunch with an exactly looking cable, laced it the same way as the other bunches, and put it kind of hidden between them. It didn’t stand out at all, most people would not notice at all.