Sometimes there is a good reason to use a more breakable plastic part. Baratza coffee grinders are sturdy and well-designed, but there is a plastic gear in the power chain between the motor and the burrs.
That gear isn't plastic because it's cheaper than a metal gear, it is a sacrificial part.
Suppose you get some coffee beans with a rock in them. The burrs seize up because they can't grind the rock, and you don't notice it in time to cut the power.
Imagine that every part in the power chain is as sturdy as the motor and the burrs. The motor may burn out, or else it manages to force the burrs to turn, ruining them.
Now you have to replace either the expensive motor or the expensive burrs.
Instead, the plastic gear fails, saving the motor and burrs. This gear is cheap and easy to replace. (Baratza may send you one for free, even if the machine is out of warranty. Their customer service is second to none.)
So there are some cases where a breakable plastic part can avoid damaging or ruining the more expensive parts.
Of course one could imagine other ways of solving the "grinding a rock" problem. Maybe some kind of sensor to turn off the motor if it seizes up? But that would increase the cost of the grinder, and who knows if it may have other failure modes. Since this is such a rare situation, the sacrificial plastic gear is a simple and effective solution.
When I was a kid, our family went to a drive-in theater in the family station wagon. I decided to watch by sticking my head out the back window. My dad decided to close the window; thankfully he realized my head was there before he finished. I'm glad I never got to find out if there was a plastic gear in the mechanism.
That gear isn't plastic because it's cheaper than a metal gear, it is a sacrificial part.
Suppose you get some coffee beans with a rock in them. The burrs seize up because they can't grind the rock, and you don't notice it in time to cut the power.
Imagine that every part in the power chain is as sturdy as the motor and the burrs. The motor may burn out, or else it manages to force the burrs to turn, ruining them.
Now you have to replace either the expensive motor or the expensive burrs.
Instead, the plastic gear fails, saving the motor and burrs. This gear is cheap and easy to replace. (Baratza may send you one for free, even if the machine is out of warranty. Their customer service is second to none.)
So there are some cases where a breakable plastic part can avoid damaging or ruining the more expensive parts.
Of course one could imagine other ways of solving the "grinding a rock" problem. Maybe some kind of sensor to turn off the motor if it seizes up? But that would increase the cost of the grinder, and who knows if it may have other failure modes. Since this is such a rare situation, the sacrificial plastic gear is a simple and effective solution.