I broke three shift cables with DA 7800 shifters. Finally got the shifter replaced under warranty.

Just broke another shift cable recentl with DA 7900 shifters.

On my commuter with 105 levers, I replaced the cables when I noticed the shifting getting really bad. The cable was badly frayed. It was hanging on by maybe 1/3rd of the original strands.

Yes, but the high and low limits on the indexing are set at the derailleur, which means that you can strain the cable against them.

I bet people that constantly "run out of low gears" and try to shift against the limit strain their cable moreso than those who don't.

that's like 25 years of riding for most bike owners

My right shifter cable broke at about 3500 miles (Shimano 105 - 10-speed). A few hundred miles later the crosspiece the the brake cable threads through broke on the same side. The shift cable I understand since it gets used plenty. The brake thing not so much. My only thought is that it's a silly little piece of plastic and it just wore out.

I will now make it a point to replace cables each winter. Breaking a cable ruined the chance for a good day of riding.

My Campy cables will start to cause a decrease in shifting performance long before any apparent sign of fraying/ imminent breakage.

I replace the entire set of brake/shifter cables and housings at the beginning of each year. I think I pay $45 online for the factory set.

I replace mine every couple years prophylactically or when I notice any cable fraying, whichever comes first.

I change my shifting cables every 5000-6000 miles. I have removed them and found a few strands already broken and frayed.

This is a good time then to relube the inside of the cable housing. I have replaced cables and found them to be rusted inside the housing and not sliding very smoothly.