But that's the crux of engineering: these two things are in tension – the part which is designed to fail safely if a jam occurs will, by its very nature, also be subject to wearing down sooner than if it were made of cast iron and caused the entire mixer to fail catastrophically during a jam.

In reality, nothing is so simple as waving your hands and declaring "no plastic 'planned obsolescence' parts!". Everything is a trade-off. A more effective way of pushing companies to position the trade-off in a way that rewards long-term durability would probably be to mandate longer warranty periods, rather than try to dictate what gears ought to be made out of in a vacuum divorced from the tradeoffs involved.