FNAL, BNL, CERN ...and KEK all use radhard electronics (obviously not everywhere), admittedly on the fringes of your illustrating application areas. Combined, that is already quite a large area, so to my mind parts for tough (damage) control robotics isn't such a niche.

But true, small production runs and spares is a real issue (actually, by the time they are really used, they are already obsolete -- but this is why you have upgrade plans :).