Don't know what has happened to all the good technicians but this is what I remember from the late 1970s.
A lot of the bigger arcades in Sydney (with 30 or more pinball machines) had one or two parlour owner/operators on site at all times. They usually cleaned playfields on a weekly basis and replaced broken rubbers or blown bulbs. The glass top was cleaned on a daily basis as there was a lot of tar and soot from cigarettes as most players were smokers and had the fag resting on the glass top. These guys knew that well maintained machines brought in more players with their cash.
A qualified technician usually came once a week as part of his rounds and repaired any major faults that reqired electrical or mechanical skills. He was on call as well and it was rare to see an out of order sign on a machine for more than 24 hours.
Ah, the good old days of the pinball golden era.