Maybe 5 years ago. Planes today are full, even in international F, after all the trimming they did to cut cost by reducing routes and frequency to match or undercut demand. My 4x F cabins last month were booked up, so much so that they offered me two days later on a cancellation because the next day was full, and I was the first in line.

So I doubt they're selling seats at a loss. Particularly NA airlines. I haven't been on a single plane this year that was below 50% capacity. The ones less than 70% were red-eye 4-6hr flights.