On that basis I am surprised that they are not developing the 778 and 779 already. Resting on one's laurels is never the best business strategy in this industry!
Airbus slipping the in-service date of the A350-1000 by two years to 2018 gave them a breathing space a) to re-build the R&D war chest following 787 and 748 development and b) to see what the engine manufacturers could do given a few more years. With the additional time and resources they can comtemplate a brand-new carbon wing and brand-new engines (as opposed to further developments of the GE90 and/or Trent). In-service date is now 2020 at the earliest and possibly a bit later i.e. at least two to three years behind the -1000 but Boeing reckon they will have another generation of engine technology which, with the new wing, will ensure Emirates and others will wait for. It also lets them complete what they see as the greater priorities of 737 MAX development and stretch of the 787 to the 787-10 ahead of re-doing the 777.