I already have several bicycles with plastic rear derailleurs. They also have plastic crank arms. And lots of other bits and pieces made of "plastic".

In case you didn't guess, "plastic" is not a good way to describe something. Different plastics have very different properties. Some are very good for making high end bicycle components.

Almost all of the bottle cages on my bicycles are "plastic" with most being some kind of nylon. Much better than the old aluminum cages and much cheaper and better than a titanium cage.

Technology has progressed a long ways from hand forged steel. It will continue to progress for the foreseeable future. Get over it.

And for the record, I don't care what my bike components are made of or what color they are. Cost effective performance is what matters.

Oh, next on my upgrade list is a CarbonTi chain ring. Not cost effective, but really good.