BKK-JFK is only slightly longer than DFW-SYD (100 miles). The longest flight then flown was actually SQ's SIN-EWR flight also flown by A345's, but reasons articulated for its cancellation in 2013 are the same. Reportedly the fuel bill was completely disproportionate (I think it allegedly made up 8% of SQ's total fuel bill, or something like was rumoured)
Maybe it will be targetted at premium end. In business, having experienced SIN-EWR and SIN-LAX or VV several times, it's just an overall more pleasant experience than waking in the middle of the night (or whatever time it may be), trudging through security, 30 mins in a lounge before trudging back to your same seat again. Gives you incredible flexibility to eat, sleep etc when you want to, not to be dictated by a layover. Even in Y, I found the JFK-HKG sector(16 hours + 1 hr ATC delay in JFK) easier than expected. Of course for those wanting to stretch their legs regularly, there's always EK's three hop services to Europe via SIN, KUL or BKK & DXB.