There are only a select handfull in this hobby that actually profit enough from it to make a living doing it. The parts businesses are more or less under an "all encompassing" service. Some guy/gal that lives in a remote part of the country is probably buying stuff from parts places left and right these days... even if they only have one or two games.
And most often with the pinball businesses that I encounter that are making some decent $, they also have other forms of income as well. They route games or are custom cabinet or furniture manufacturers, own a plating business, are artists that touchup/clear-coat on the side, or are in the automative painting business... A LOT of these people were professional artists long before they started making pins look new again.
I hate the car-gument but in this case, it's very similar. You can make a LOT more money as a certified diesel mechanic (per week) than you could ever make restoring classic muscle cars. There are exceptions to this rule (just like in pinball) but you would need some serious coin AND TALENT to start up a classic automative restoration business these days.
Pinball is a much smaller scale but no one just wakes up one day and decides to start restoring vintage arcade or pinball games for a living. You have to be passionate about it and not be all that concerned with the overall profit off any given machine, if you are doing things the right way.