I think it is worth noting that Singapore Airlines used to have a 19-hour flight from Singapore to NYC, which it flew for 9 years. Having been on this flight, the experience wasn't terrible and there was plenty of space to walk around and spread your legs, despite it being a non-A380 or anything fancy back in 2008. There were always spaces in the back of the plane with snacks and drinks where passengers could relax and look out the window and talk to the stewardesses.
But the flight was first premium economy/business only, then converted to business only, and then cancelled entirely because they couldn't make the economics work out.
Yup, they figured the two-engine plane will be more efficient than the old quad-jet A340. But this time, too, there is no standard economy, only premium eco & business.
https://www.cnbc.com/2018/06/02/singapore-airlines-is-bringi...