In my time I've essentially only owned 3 markers. An Etek1, Ego7 and Ego8.
The Etek had approximately 90,000 balls through it, and the Ego7 about 130,000 by the time I sold them and they both ran perfectly. With both they were serviced regularly by the previous owners and myself, and I just made sure I kept it all well oiled and after a days play would strip and clean it if I had taken hits to the marker or it had got wet/muddy. If it was clean on the outside I'd leave it and not strip it down.
The Ego8 was bought with under 20,000 balls through it and was an absolute dog. I don't think the previous owner had serviced it at all, and first time I took it apart it was caked with crap on the inside, orings were wearing out, it felt like it had never been oiled, and in general was a wreck.
So I don't believe that shot counts really matter at all, what matters is how well a marker has been looked after and maintained.