Many kinds of plastics can't be recycled at all, not even downcycled. Thermoset plastics particularly, which includes almost everything made out of fiberglass (glass fibers in thermoset plastic matrix.) That stuff either gets thrown into a landfill, or ground up and burnt.
That most of it burns is part of why "go burn it to offset coal" came out so well in some of the lifecycle analysis for end of life program I'm familiar with:
Instead of having to pay to sort and process, you just grind it up and mix it in with the coal (or heave the bags in, I'm not sure as to the actual feed mechanism though I keep meaning to see if I can get a tour).
I'm not actually in the program area, but I know where to drop my bags of plastic to get them into the program (one of the recycling companies in the area of the program handles it, and I'm over there often enough that I'll toss a few of the bags in the back of the car if I'm going that way).
