If the gears wear down that badly, they are either in a place where they take too much load or it's very bad plastic material. Comparing with Elna, Bernina, and Husqvarna, they last 40 years, and often still running on the original gears. Singer had some rather badly made models after the mid 60s and we have to sort out what's worth fixing and what's not. The touch and sews are tricky that way. It's odd, because good quality plastic isn't that expensive, just a bit more expensive.