Well-noted. Plastic and nylon were both cheaper, and prone to breakage (hopefully, after new mfg warranty was expired). It was the quintissentially American answer to durable machines on the used market eating into new-machine sales figures. For decades, Singer would buy back all used machines under contract, in order to rework them in their shops, or destroy, or at their discretion refit, in order to preserve new-machine and and new-model sales. With the polymer revolution, the idea was that something essential would break after the initial warranty was expired, such that they could fuck you on the repair, or hit you for a new machine.
Economics is not about fairness, it's about tolerable cruelty.