I remember flying to DEN
in F and I was the only passenger in the cabin. Plus I was not feeling well and so I slept almost the entire flight. My entire food intake were two glasses of sparkling mineral water and some macademia nuts. Notwithstanding my non-consumption the cabin crew put in place the full drinks buffet.
There were also some other routes where I was the only pax in Lufthansa F. MUC
. Other routes always full: JFK
Tickets are bought by the categories of people you mention, although quite a number of them also use private planes. But there are artists, wealthy people on a private trip, high ranking executives (think C-level of Fortune 500 companies), partners in certain law firms or strategy consulting firms, SME business owners that go back and forth between two places a lot (e.g. weekly Munich-Shanghai return) and who do it to get some decent sleep. Plus don't forget that F tickets are a lot less expensive when bought in certain markets outside of Europe.
Plus all the mileage cowboys of LH
's own frequent flyer programme and those from US-based frequent flyer programs using upgrade vouchers.