that level of seating on a 772 would be with a mini-business class cabin so they can push the CASM down but also the revenue generating capability of the forward cabin.
It also would require 10 abreast seating, something that AA has or will have on its entire 777 fleet but UA does not have on its 772s (perhaps yet) and DL has not said it will do - and probably won't. Has UA moved forward with that high density 772 mod?
further, there is no doubt that the economics of the 787 will be favorable, esp. with the stretched -9, relative to the 777 and perhaps the 330.
But one airline believes that when the cost of ownership is factored in, including the discounts that Airbus can give on an aircraft for which they have sold 1000 copies, the 330neo is a more economical alternative but the economics of the 359 vs. the 787-9 are about the same.
Comparing aircraft doing similar mission with similar configurations is necesary in order to get reliable data.
the 777 is a great airplane... but it is heavy and not as efficient as newer aircraft including the 330, 787, and 350 and on TATL flights where the range and extra weight or technology to reduce the weight of a long-range aircraft are not needed, the 330 and even the 764 compare per seat more favorably than the 777.
that is probably why DL and UA - who both have the 764 and DL and US - who both have the 333 - rely heavily on those models instead of the 777.
Ownership costs dramatically rise if a fleet has to be early retired because there aren't enough routes that justify their operation. and if a carrier flies a heavy aircraft on a mission relative to other aircraft used by competitors, the costs will be higher.
and DL and UA are not ordering new technology aircraft faster than they need to do to replace retiring aircraft instead of shuffling older technology aircraft medium global regions.