Plastic is never a great material for parts whose primary purpose is transmission of power, especially via friction (so gears and such). It's great for parts that don't have to withstand any force, but unfortunately it increasingly gets used for parts that do.

When you make the motor, including the shaft itself, out of metal, saving 2 grams by making a cog out of the cheapest ~~chocolate~~ plastic available isn't saving much weight or money. It does however guarantee the device will fail earlier.

You can also have a more efficient system without sacrificing durability. Take a look at the Bosch CP3 high pressure fuel pump. Stout metal piston. Million kilometers is not unheard of. Meanwhile, Continental/Siemens made a hpfp that used super thin membranes to pump fuel. Those usually die in less than 100k km. Both available at the same time and both serve the goal of enabling using direct injection to improve fuel efficiency and emissions. But the joke here is that the Bosch one runs at much higher pressures (it's for diesels) and is STILL cheaper to buy new. So people with engines that use the Siemens one would just scrap the car if it goes and they can't get a specific used piston pump to replace it with (also a Bosch one, but I forgot the name). This is in the GM 2.2 Direct engine in Vauxhalls and Opels. Those cars have disposable fueling systems (and engines in fact since you can melt a piston once that membrane pump fails).