What will happen in the future when parts may not be available. I can only speak of one maker with any certainty. Mr. Dufour had a Japanese watchmaker in his shop when we visited last summer. He was just finishing his time there where he learned to be the official repairer for the Asian market (sorry going from memory here so hopefully I have all this correct). I would guess the next best thing to having your watch serviced by the one who created it, is having someone directly trained by that person doing the work.
As far as what is "typically" changed at a service....the only thing that comes to mind off the top of my head is the main spring. Not all watchmakers do this as a matter of course either. My guess (only a guess) is that the mainsprings can be bought from any supply house by using the width, thickness, length and end type.
Main spring aside, unless other parts are damaged due to some shock or other unfortunate incident, then a cleaning, oiling, and adjustment are all that is requried.
Cheers, Al