Before I went to Di2 on my main bike, I replaced the rear shift cable every 6 months (about 2,500 miles) and front every year. Simple job when the cable is still undamaged. Absolute PITA once the cable has started shredding inside the shifter.
Cables aren't very expensive, it's not worth waiting until they fail. Not only does this leave you in a bind when it happens mid-ride. Picking the little bits of broken cable out of the shifter is tedious.

Berd shift cables, bro!!!!

I'll add a tip to make your upcoming inspections easier, but it only works if your cable housing is routed through a slotted frame cable stop. Shift into the largest sprocket (front or back), stop pedaling, then shift into the smallest sprocket position. This will give enough slack to pull the housing out of the frame stop. Squeeze the brake lever to expose the cable entrance hole. Push the cable out of the shifter to inspect for broken strands. This way you can retain all shifting adjustments.

Also, if you find yourself turning a barrel adjuster every few weeks to keep shifting smoothly, that's a sign the cable is fraying.

The OP discussed the issue is with a Pinion Gearbox, with TRP Hylex levers… please let me know if your tip applies to this system.

Good point about the gearbox--I don't know if you can shift to high and get the gearbox to not shift back into low. Probably not. And it may be simpler to just disconnect the cable and push it out of the shifter.

I suspect not, as the Pinion uses a pair of counter-pulling cables. I'm not familiar with that shift lever though.

thigh-- have seen you over on the touring section.
Curious--does one brifter shift up and the other side down?
Is the worn side for upshifts?

specifically about tension--Ive had this happen on my spd era shimano Tiagra brifters, but I also noticed that in my case, it has been worse when slight derailleur adjustments were off a bit, making more tension for cable, increasing tension at this crucial bend point, and this tipped the balance for increased wear.
Once I figured this out, I've had much better cable life happen.

I don't know if a pinion setup can be slightly misadjusted and cause more cable pull tension that is really needed? Perhaps look into this.
I also very strongly second or thirdly suggest inspecting more often. With a rd, in hindsight I also realized that I had been ignoring clues, ie rear shifting getting a bit sloppy and then having to do a teeny tiny barrel adjuster turn to remedy -- this was the very start of fraying. I've been lucky and have been able to remove the frayed bits easily to replace.

I wonder also about your housing, it is full length like on my tough touring bike, a Surly Troll set up with Genevalle shifters (look them up) and full length housings can have more friction also, so what about tri flow or silicone stuff in the housings?
You should be able to get a feel for the force needed to move your left shifter and see if you can noticeably reduce the force with various changes.

are you sure you aren't shifting under load , or thereabouts?
Are you hard on things in general?

as you say, it's discouraging that after 5000 kms this has occurred, so clearly if you do not find a specific reason that can reduce the tension, you'll have to keep track of cable life and pre-empt a problem.
I wonder if a silcone spray lube (clean, non sticky, but very slippery) on the bend area could help things long term? I use this stuff in bike and house stuff and am always amazed by how it reduces friction--I use it on my old 9 spd brifters, my old 90s mtb front suspension fork to drastically reduce fork stiction on this very old simple fork , its my canadian winter bike; on sliding windows in the house (amazing how easier they slide after, and for a long time)

Here we have a product , spray can, called Jig-a-Loo, you can look it up and see its specific properties. Im sure there are similar sprays where you are in the States.

good luck, I'm curious to what you can find out

I do this inspection on many of the service write ups I do, preferably in front of the customer. You should see their eyes open up if the cable is starting to fray... I've taught this to so many mechanics (sadly including some that were "experienced" ones) as a way to show the customer their bike and your understanding of it and how you assess the bike before quoting costs. Andy.

thanks for your ideas… I have learned the tension may have been a bit high on the opposing shifter. I am interested in lubrication options, for now I am using coated, higher quality cables and loosening the lever tension.

good luck finding a solution or improvement
I'm still curious, does one side brifter shift up and the other shift down? Your setup is pretty rare, I've just read of grip type shifters for rohlofs and pinions (although I've seen stuff about the electronic pinion shifters)

Dyneema spokes have been a thing for a few years, they receive enthusiastic reviews and seem to last well (they may need occasional retensioning due to creep - ultimately this could lead to failure, but I've not seen any reports of failure not due to accidental damage).

Thanks! I'd searched for Dyneema spokes after seeing LV2TNDM's post back on the 4th but came up empty.

Seems they're not 'there' just yet, but maybe someday?

https://berdspokes.com

Another search just now brought me these:

https://www.dyneema.com/sectors/spor...h-berd-spokes/


(There's more there too but I'll leave it to other interested parties to look into by theyselves... only caveat? Be sure you're sitting down when you get to what Berd is asking for their spokes. This stuff is most assuredly on the bleeding edge of bike tech.)

The right lever thrifts down, the left shifts up.