There is certainly a SMALL market of people who DO PAY for F. Some people (the sort who waste money buy Rolls Royces and Astons) would rather pay $5k extra for a 'marginally' better seat than not be the king of the hill and have to sit in business with the riff-raff.

I think most of us wouldn't consider spending that kind of money... but there are people who do. In addition, it lets not forget that in reality, these days there is a marginal difference in cost in terms of floor space and catering (especially on UA...)... They can oversell C (at $$$$ rates) and offer mileage upgrades/SWUs to people who spend on full fare (C/D) rather than just Z fares. Don't forget that CO sells most of its BF cabin at cut rate Z fares, sort of like they do domestically.