I have found the breaking of shift cables an interesting topic. Why have I never broken one?
What I have learned is Shimano shifters put the shift cable into a very tight radius within the shifter. This causes more stress and fatigue on the cable.
I own 8 speed STI Dura Ace and Ultegra and 7800 Dura Ace 10 speed but don't use them much anymore. I went to the dark side (SRAM Red 11 speed and then e-Tap). I always attributed my good fortune with no breakage to replacing the cable periodically (as a preventative). Now that I have e-Tap, my shift count is recorded on the Garmin and I have learned that I shift much less frequently compared to other cyclists. I also read that 2,500-3,000 miles is about how long the right shifter cable lasts on many Shimano equipped rider's bikes or at least that is what I read. Assuming I shift 1/3 the frequency of most riders, I might have been able to get 7,500-9,000 miles out of a shifter cable. I used to just remove the inner cable, slide in a new cable into the same housing. It isn't a housing problem and I was not going to buy a $55 cable set to replace a $8-10 cable. A couple times per year and I never had one even frayed. I did use coated cables (Shimano, mid-tier Jagwire, no-name on Amazon) but it is doubtful that matters a great deal in terms of the strands breaking.
I guess if your RD cable broke and could not repair in the field, you'd have to convert to a single gear maybe wedging something into the rear derailleur to get it positioned somewhere in the middle of the cassette.