Last week we had a rash of bikes that had developed mysterious shifting problems. Adjusted derailleurs would creep out of adjustment a little more with every shift.
fraying inside the shifter, which can also have expensive consequences. A piece of the plastic liner of the housing has been shoved all the way through the shifter.
If you keep having to adjust your derailleur cable and take more and more cable through the anchor bolt, undo the cable and pull the housing out of all the stops along its route.
Five millimeter housing with beefy metal end caps is the least likely to fail in this manner. Cheesy OEM 4mm with plastic caps is the most likely. You decide.