I suspect the plastic gears suffer more from heavy use than normal use. My wife has a 538 she got in 78 or so. We got married in 83 and she broke the first set of feed dog gears not long after that. She was making teddy bears, and other sewn crafts as well as wedding gowns and even a seat reupholster for my old Chevy. Since then she's had two other sets of those gears replaced.
Those were the old straight cut gears and they now have spiral cut gears as replacements that are supposed to be better and more durable. We are going to replace all the gears in her 538 as soon as she allots the funds to get the parts. Hopefully that will give her original machine back to her and she can use it again.
Joe