The most likely causes are cable friction and derailleur hanger misalignment. Indexing adjustment has gotten more sensitive to both as the speed count has increased.
Derailleur hanger misalignment will tend to reduce the margin for error in the adjustment, and make it hard to avoid certain gears where there's some jumping or noise. When it's aligned, the rear derailleur adjustment might have a range of about one full turn of the barrel adjuster where it will work even if the shifting is more laggy in one direction than the other. Some misalignment might make it so there's only one quarter-turn "notch" position where it works, which in turn means any small acquired slack etc in the cable will require readjustment.
A lot of road bikes, including expensive ones, now have bad cable routing that makes it difficult to avoid high friction spots. Cable friction also contributes to situations where you can barely find an adjustment where shifting works in both directions, which then makes it slip out of adustment that much easier.
At 30,000km and with regular service I assume the shop has done some kind of replacing of cables and housings already. (Edit: I see you say it's still on the original cables. If that's true then yes you need to just replace them, but they wouldn't usually last that long at the best of times so I'm not so sure.) Sometimes what can happen on bikes with routing friction issues, particularly on modern road bikes with all sorts of internal routing schemes, is that new cables and housings (particularly coated cables) are enough to temporarily overcome friction being caused by some aspect of the cable setup or routing, but it doesn't last. Sometimes it's necessary to go off-script with the routing setup in situations like that.
A final thing to double check: if the housings aren't properly seated in the levers under the tape, it can be an invisible problem that makes it impossible for an adjustment to hold, as the effective length of that piece of housing is unstable.
You can check and see if it's friction by disconnecting the cable anchor and simply pulling on the cable gently where it approaches the anchor as you work the shifter. It should be very low-resistance.