NRT slots operate under a "use it or lose it" rule. This one reason why UA has , in the past, juggled the number of NRT - ICN flights: they want to keep a slot occupied while say, a second LAX - NRT is dormant during low season.

NRT slots (unlike LHR slots) can not be sold. If vacated, they fall back into an allocation pool (i.e. AA's SJC - NRT). NRT slots are hard to come by.

UA runs 2x ORD - NRT. A single NH 773 will not supplant the capacity offered by 2x UA frequency, although it may cause one frequency to end or an equipment downgauge.

Ceasing JFK - NRT or SEA - NRT may be a good thing, as it will free aircraft and NRT slot(s) to fly a route such as NRT - India. This is good, because you could use a single SWU to get from North America to India instead of wasting two when you switch planes in FRA/MUC on LH.

"Hey, what about NRT - MNL": too much VFR traffic. Also, if you think South America is non-rev heaven....

"UA should get some 787s to reduce their reliance upon NRT". Not going to happen anytime soon.

You can not reduce too much North American feed into NRT. Otherwise, you may not have enough pax to feed onto onwards flights. Then, you'll become dependent on codeshare, partner, and O&D traffic.

Yes, I agree with lucius and think ORD - KIX could be on the chopping block, considering its previous downgauge from daily to 3x weekly service.