On the one hand, they now have plastic grinding gears in disposable pepper grinders, on the other I recently hosed out a Rubbermaid bucket and discovered that I could break pieces off it with my fingers -- and it hasn't even been outside all that much.
On balance, I decry plastic parts for sewing machines because you can't be sure something will last eighty years when you have no eighty-year-old samples. My White Family Rotary is over a hundred years old and all the steel parts are in perfect working order.
But today I was trying to zig-zag along drawn threads in black fabric. I could really go for a machine that had a plastic needle plate with a light under it!