Lots of people in Y would rather break their journey in LAX anyway. While the US border is painful in LAX, JFK aint exactly great either.
Do you have any data on the one-stop preference to NYC in Y, especially when that one stop is in LAX? I've never done LAX, but agree that JFK immigration was an interesting introduction to run-down US infrastructure.
One-stop I've chosen Japan to the east coast, and the old connections gave a travel time just over 23 hours SYD-JFK or BOS. BOS is a far nicer entry port than JFK. HKG last trip was done on price and availability (transit was still fine) but took the trip over 26 hours. At least both options avoided US domestic transcon.
I agree that ULH does seem niche, but likely high yield to JFK and LHR.