Candace,
In this instance, I've got to disagree with you. I've seen way too many things over the years that transitioned from having metal parts to plastic, and yes nylon is a form of plastic, and always the quality and longevity goes down the crapper.
I will not argue that Berninas are far far better than the machines Singer sells. But the reason for casting plastic parts is cost.
To machine the cam stack for the Necchi that started this thread from steel would take a major amount of machining, assembly, and fitting. They can create a mold and cast it out of plastic for cheep cheep cheep. Then charge outrageous prices for it. I'll bet that $169.99 cam stack didn't cost any more than $5.00 to make and I'd include the designing and making of the mold as well.
There are earlier Singers that share the same design yet some use steel gears whereas the other ones use plastic gears. Same exact designs, just the difference in the gears. The reason, cost. A plastic gear ( nylon if you insist ) can be cast for pennies whereas the steel gears must be machined and fitted. That is the bottom line, cost.
One more thing. Starting in the early 60s American manufacturing went though a rethinking process that cheapened almost everything. Parts that had been previously forged and machined where now stamped or cast. This occurred across American industry from the sewing machine industry to firearms, cars, electric fans, you name it.
Look at 50s or earlier sewing machine and you'll find machined parts. Rarely will you see a stamped part. Then in the 60s machines went to stamped metal parts and yes, plastic gears. Just pop the top on a "modern" machine and what do you see?
Everything except rods or shafts will be stamped or plastic. You can stamp metal cheaply and you can cast plastic really cheap.
It's all cost related. It doesn't matter if it's a Bernina or a Chinese made Singer. Gears are complicated to machine, so they use plastic cos they are cheap.
Joe