1) Most mechanical cameras normally don't need new "parts" (apart from grease and foam every few decades), unless they get dropped or otherwise smashed.
2) When mechanical cameras do die, it's for a variety of reasons, so the "parts" cameras repairpersons keep handy can usually donate what's needed (unlike electronic cameras, where the "parts" camera probably died from the same electronic failure the camera to be repaired did).
3) Expertise... I rather quickly taught myself to do some fairly complicated repairs on mechanical cameras. I wouldn't even try to reverse-engineer a circuit board!
4) You evidently haven't used/handled many Nikon F or Spotmatic bodies (or a large number of other good mechanical cameras). Everytime I see one in a shop or flea market, I give it a quick check: the vast majority are in good working order (and my sample is probably well over on thousand). Most electronic cameras have a far lower percentage of "still-working" samples still around.
But if you choose to stick to your obviously deeply cherished beliefs despite empirical evidence and many people's real-life experience to the contrary, go ahead....