One Flight International journalist hypothesized a few months ago that potential customers are actually waiting to see more of the 777x (actual performance figures etc.) before making a decision between the 350-1000 or the 777-8. The-9 is actually alone in its 400+ seat market and replaces no previous 777 product. Once the 777X actually flies, it would unlock the 365 seat widebody market. (350-900 vs. 777-8)
What's puzzling to me is that Airbus was forced to build the 330-Neo because customers wanted a cheaper version than the 350-800. Yet, nobody is buying it. The ironic part is that the first A350 design would have actually been very similar to the 330-Neo. Until this was also trashed because of customer requests.