Airbus (A) and Boeing (B) each have dogs in the wide-body fight. Airbus is seeking a packaged deal with its A-350-900, 1000, and the 1100 proposal. Boeing is going to the mat with its 787-9 and 787-10. Don't count the 777X out of the equation at this point. Perhaps Emirates stalling on making a decision is waiting for the 777-9X to fly around the world during tests.
My own belief, this is the "trump" Boeing card which wishes to play hard going against Airbus with a knock-down drag-out fight over time. However, during 2017 is going to be an exciting year for Boeing with its testing of a new family of wide body such as the 777-8X and 777-9X where sometimes is forgotten by aviation watchers from around the journalist world watching for 787 orders.
When considering the 787-9 and 787-10 against the Airbus A-350 family you must throw in the 777-X's as a Boeing counter offer. Emirates is anxious to watch what the 777-X will do before dropping serious coin into an early order hat. That comes next year so they must keep the suitors of Airplane sales in check a little while longer. The 777X deliveries will start in 2020. A time period which will suite any buyer in hot pursuit of an extreme wide-body going 9,000 miles.
Distance is a key issue for Boeing as it offers its 787-10's for medium haul. They travel about 6,800 miles on an 10's extended route while the 777-8X or -9X which is designed to go plus 9,000 miles mops up the global market. The choice for Emirates will happen during testing of the 777-9X at Paine Field Everett, Washington.
If Boeing validates the 777X concept through various test flights, then Emirates will order a bevy of Boeing Aircraft late 2017. The order made will include the approximations of Boeing types fitting Emirates needs. As follows a Guesstiment: 20 787-9, 10 787-10, 10 777-8X and finally 15 777-9X which opens up for options of about 50 more wide bodies going forward. Boeing has about 105 wide-bodies in play replacing the 70 cancelled A-350's from 2014.
Emirates will need to hurry as the 787 production window is beginning to fill up with orders from Qatar, China Southern and others for the 787-9's. A move by Emirates may certainly cancel some A-380's in waiting as the Boeing family of aircraft will fill multi segments of Emirates market with right sized aircraft. The Euro -American sector would nicely fit the 787-10 while the Emirates- East Asia market is built for the 787-9. Finally, all circumvention of the globe is left to the 777-X's with 350-400 passengers.