The 777-X has the same 17.2 "seats as the 787, It does not want to gain 2 cm as you claim.
Literally from an article pinned on Boeing's 777X page:
By redesigning the sidewalls and fitting thinner insulation, internal cabin width has been carved out from the 231in (587cm) of the B777-300ER, to 235in (597cm). Four inches may not sound like much, but it represents a significant engineering and design effort, and that extra width gives airlines more design flexibility and scope for expression – something that airline customers have indicated to Boeing that they want...
....that wide cabin enables 18in-wide seats at 10 abreast in economy.
https://www.aircraftinteriorsinternatio ... o-far.html
And that article is pinned on Boeing's 777X page.
The 787 has a cabin for 7-abreast Premium seat and not 8-abreast.
Depends on carrier. My point is that a passenger shouldn't have to pay W fares to get an 18" seat. Airbus has this as standard on every mainline carrier. Boeing should be able to pull it off.