UPDATE: Thank you for the comment! Of course I picked a wrong MTOW for the 777-300ER. I corrected it and the values coming out of it. The basic conclusion keeps the same, especially as the announced SFC reduction of the GE9X will be 10% rather than 7%.
Of course, even with the small increase in inner diameter of the B777-X a ten-abreast seating will be less comfortable than a 9 abreast-seating in the A350. And more capacity is always higher risk (for revenue vs. costs), but for all current B747-400 routes the B777-9 would be the ideal replacement. The A350-1000 will fly with lower costs per flight, so if you can fill the A350-1000 it will be a profit maker as well.
The proposed B777-X models, the larger B777-9 holding about 405 passengers and the B777-8 with about 350 seats, should slash fuel burn per seat by 15% (B777-9 vs. B777-300ER) and about 10% (B777-8 vs. B777-200LR).
Is that possible? Let's have a look by taking a very simple approach and concentrate on the comparison