It's possible flights are booked out given the relatively small seating capacity of the QFi B789, especially as configured on this long route, but try as I might for a 'ghost' booking on ITA Matrix, for a day in September 2023 when QFi operates QF3 and in October 2023 back on QF4, I couldn't get ITA Matrix to bring up this routing.
The cheapest J return fare it offered was an outrageous A$19529 on QFi, taking 25 hours 39 minutes (gotta love the accuracy!) outbound and 25'25" inbound from JFK. This used AA flight numbers AA4111, AA274 and AA185 and then old stalwart QF12. Travel was via LAX.
I assume fares on QF3/QF4 will be higher than via LAX.
OZ had a J class fare via ICN of A$7872 return respectively taking 27'30" and 28'35". NH via HND cost A$8449 and was scheduled to occupy 28' and 27'15". PR at A$8086 was 26'30" oubound but slower at 41'10" returning, allowing about 15 hours in MNL, which may suit some as it breaks a long journey. I had to tweak ITA Matrix for the latter by allowing plus or minus two days as PR may not operate MNL-JFK and return on the same days as QFi.
There were other options including KE via ICN that were substantially less expensive than overpriced QFi.
Many who have QFF points and redeem (if one can ever obtain seats), those who want to accumulate more points or have travel paid for by their company or government department/agency will travel on QFi, but if paying out of one's own pocket, why would you use this route given excellent alternatives that either only take a few hours longer in each direction, or offer the opportunity for a day's (or longer) stopover?
It's an enormous premium for not many hours' saving and arguably poorer service than on competitor airlines given how so many suggest QFi has declined. (Read the scathing letter in 'AFR' today from a gent who lives in BrisVegas: he recently travelled on A330s, admittedly a different aircraft to newer B789s).