If in fact the 78X is slightly more capable than the 777ER, it would seem to me there are only a handful of flights where the 78X can’t cover the range, and most of them don’t need the additional seats over the 789. BTW, a few of those cited were stretches for the 777ER at certain times of the year I believe
The 78X is not more capable than a 77E once you cross into the long haul routes and I'll do my best to try and explain but I am by no means an expert. First I'll start with both the 789 and 78X both of these aircraft have the same MTOG at 560,000 LBS, and the same max fuel capacity which is 226,900 LBS (this does vary depending on density) . However the 78X does have a higher OEW at 300,429 LBS vs the 789 at 288,210 and a higher MZFW at 425,000LBS vs 400,000 LBS for the 789. (These OEW numbers are actual real world numbers models with Polaris/PE seats not Boeing's generic numbers.)
One of the issues with the 78X is if you use all 425,000 LBS of ZFW you will have to restricted the TOG. For instance if you use all 425,000 LBS of ZFW you would need to restrict the TOG to 535,774, and vice versa if you go with MTOG at 560,000 your ZFW is limited to 396,700.
So what does this mean? It means the longer the flight, the more fuel required to operate the flight the less ZFW you have available. However shorter flight let's say with-in that 7-9 hour range, you can operate at up to MZFW but with a reduced TOG. Any flight shorter than 7 hours can operate at both MZFW and MTOG because the required fuel load is so low but the chances of you hitting MTOG on a short flight is little to none because you will run out of ZFW before you hit MTOG.
Take TLV-EWR an 11 hour flight, that route normally needs MTOG at 560,000 LBS but the available ZFW is restricted to only 396,700 with an average gate fuel around 164,000 LBS. Take out the taxi fuel normally around 1,000 LBS your cleared fuel is 163,000 LBs
United's 78Xs have 318 passenger that translates into 62,010 LBS if we used standard winter weight of 195 LBS per passenger (the summer weight is 190 LBS per pax).
A lot of these long haul routes normally get around 500 bags ( a few are heavy bags) but lets say no heavy bags that equals 15,000 LBS at 30 LBS per bag.
So we have OEW 300,429 LBS + pax 62,010 LBS + bags 15,000 LBS = 377,439 LBS this is your ZFW.
Now on a route like TLV-EWR United still has ZFW remaining to we can take cargo 19,261 LBS if the flight is 100% full with 500 bags.
However as stage length get longer and the required fuel load get higher the ZFW has to be restricted even more on a 78X. So at an 11 hour stage length we are at 396,700 LBS ZFW. SFO-AKL another route UA used the 78X on had a stage lenght around 12.5 hours the available ZFW never exceeded 383,500 LBS ZFW. However out SFO-AKL flights would normally exceed 500 bags because leisure routes normally have a higher bag count than EWR-TLV-EWR which is a leisure but also a business route for United Airlines. Also on SFO-AKL-SFO United could not carry any cargo on the 78X there wasn't much ZFW remaining.
Now compare that to the 77E which could handle a 14+ hour flight with a full passenger cabin, plus all their bags, and cargo. Even though UA's 77Es accommodate less passengers at 273 United could still carry cargo on the 77E when that aircraft was on our SFO-AKL-SFO route. The 77E when operating at or near MTOG has a higher available ZFW when compared to a 78X.
I think part of the problem with the 78X comes down to the size of the wings and relatively small wing tanks compared to the 77E. The 78X with a wing span almost identical to the 77E can only hold 36,550 LBS of fuel in each wing so that is around 73,100 LBS of fuel in the wings anything above that amount has to go in the center tank. Compare that to the 77Es whose wings can hold 62,850 LBS of fuel which equals 125,700 LBS of fuel in the wings. The smaller wing tanks on the 78X effect the available ZFW as your flight time increases.
The 78X in its current form is max out, if Boeing is going to increase the capability of the 78X the aircraft at the very least will need a higher MTOG which would require larger wing, larger wing tanks, a stronger wing box, and larger landing gear. This isn't a simply tweak, we are talking a major redesign like what we are seeing Boeing do with the 777-9 vs the 77W and not what Boeing did to turn the 77A into the 77E. I have no way to compare a 78X to an A359 in the real world but it is clear that as the flight time and fuel increases the pendulum begins to swing in favor of the 77E or the 789. The sweet spot for the 78X is any flight with a flight time less than 11.5 hours because the 78X is more capable than a 77E on those routes. Anything higher than 11.5 hours you going to run into problems especially if your talking about heavy cargo route.