That's not quite true AFAIK at least a couple of years ago the super long routes (18+h) were served in business class only because not enough customers wanted to do such a long flight in economy.
The Singapore Airlines Singapore to New York flight is 19 hours and only has premium economy and business. I have to make this trip fairly often and I prefer to take the direct flight as it’s simpler, faster and I have to pay extra for leg room either way. That said, if I had the means I would take business each time, because 19 hours in even the nicest economy seat can be tough.
Another factor besides comfort is that they can extend the flight time 1-2 hours by having fewer passengers and bags onboard, which of course means premium seats like SQ23/24.
Qantas is planning to launch 18–21 hour routes in 2026 with economy class, using modified A350-1000 with an extra fuel tank in the hold.
That's mostly not true as far as I've seen because most trans-pacific flyers are not willing/able to spend thousands of additional $ to fly in business class.
