> With the existence of economy-only planes and lack of 1st-class only planes, I’d argue that the “cheap seats” are what makes aviation viable, rather than being a money-loser.
On a typical flight, the revenue probably follows a Pareto distribution--20% of the passengers (first class) provide 80% of the revenue--but at the paper-thin margins of an airline, you still need the other 20%.
Economy-only flights are usually from economy-only carriers, which trade even thinner margins for not having to worry about the annoyances of catering to first class passengers.
Economy conditions are increasingly terrible because cost-sensitivity and easy-to-verify sources of pain (nonstop vs. layover flights) have a greater effect on consumer behavior than seat size and other creature comforts. With these conditions getting increasingly worse, economy flights become increasingly more cost-effective, and perhaps it becomes increasingly harder to keep the pampered first class passengers on the same airline or even the same plane as the economy passengers.
Also, first-class tickets aren't even at the very top of the market any more than Audi and Lexus are at the top of the automobile market. The top of the market is in charters and private jets. An airline that flies planes only for first class passengers is close enough to a charter airline that they might as well become a charter airline and bypass TSA and the other associated miseries of air travel.