The NYC route has nothing to do with dropping LAX-LHR at all.
NZ already sells tickets on SQ via SIN, CX via HKG to go to LHR and the rest of Europe, and even via NRT, YVR and SFO with NH, AC, VS and BA, so I suspect it will continue to focus on feeding the range of hubs available. Bear in mind that AKL-LHR is not much longer via Asia than via the US.
The LHR-LAX sector has tended to have a lot of high yielding point to point traffic over the years (but significant periods of low demand especially in economy), with the LHR-AKL traffic being less important. Of course, NZ did try to operate AKL-HKG-LHR for a few years, but the GFC killed the viability of the route, as NZ couldn't attract enough point to point traffic on LHR-HKG to make up for the crew costs, overflight costs of Russia and additional HKG airport fees. With that gone, having a LHR base of 130 staff made the single flight difficult to sustain, and with little prospect of being able to grow the route without more LHR slots, and without feeder traffic at either end (and vigorous competition from BA/AA, VS/DL, UA and DY), it was a case of focusing entirely on the Pacific.
With Air NZ deciding to go for 787-10s to replace 777-200ERs, the LHR route remained a key reason to have 777-300ERs. I wonder if they may be phased out in time for 787-10s as well, to maintain one major fleet type.