1. "Paper" cups have a pfas or other similar plastic liner.
2. Plastic cups can be easily recycled especially from a controlled environment like an airplane.
3. If you incinerate plastic cups in a Waste to Energy facility like they do in most of northern Europe, Scandanavia, Japan and Korea, the carbon footprint of plastic cups is less than that of paper.

The switch to paper cups is just virtue signalling and only works because they have dumb customers.