Some of the capacity increase on the 777X is by re-contouring the sidewalls to get a few more inches of width - combined with a slightly narrower seat the can get another seat per row in coach (going from 9 abreast to 10 abreast). So the potential is there to cram a lot of seats in coach which would give really good seat-mile costs. OTOH, much of the popularity of the current 777 with passengers is due to the relatively wide seating - which would obviously be lost with 10 abreast seating
I think you are a few years behind the times mate. Several airlines have gone the 10-abreast route on their 777s, and these days the number of 777s delivered with 10-abreast config. far outweigh those with a 9-abreast ditto. EK is the major proponent, but it is becoming the new 'standard'.
This suggests the re-contouring will either be used to reclaim some of the space lost when going to 10-abreast, or it will be used for expansion to 11-abreast.
A 777-9X at 11-abreast will sport fantastic CASM figures, but then again so would an A380 with 12-abreast down stairs and 9 upstairs. It may be possible, but it would not be a very comfortable experience for the poor sods occupying those seats for 10+ hours at a time. I've had several wanders down the back end of EK aircraft whilst enroute, and the 77Ws are utter sardine cans. One can understand why an economy-class passenger would go out of their way to get on an A380 instead, who offer much more width at shoulder and hip level as well as much wider aisles. Not to mention a much reduced noise level, but that's besides the point.
Point is, 10-abreast 77Ws are the bean counters dream machine, and as long as they rule the roost that's where the game is at.