I think the bigger question on additional 777's that aren't new build is where are they going to get the feed stock from? Boeing and Airbus are in kind of this weird dance no one really talks about in recent years with respect to all new aircraft. They really have only been directly competing with variants of the 737/320. Boeing went with the 787-8 to replace 763's and compete with the derivative A330-200 vs competing with the 380. Airbus went to the A350 which is really a 777-200/200LR replacement in the initial versions & Boeing's 777x are initially targeted to be bigger than the 350-800/900. The yet to be built A350-1000 vs the 777x in the 2020's looks like the first time they'll be competing head to head with truly identical products followed but the proposed 777x model designed to compete with the 380.

So the 777x doesn't come out to 2020-2021 and is really designed as a bigger aircraft as Boeing is still trying to coax 777-200LR orders out of airlines with early model exchanges that would provide the freighter stock. But they don't want to give discounts on an airplane that doesn't stack up as well vs the A350-800/900. So unless we're talking A350 airlines who are willing to sell their earlier model 777's, who's giving up big numbers of 777's before they acquire 777x's or later planned 787 stretches toward the 2020's? Word is that Boeing's attempt to not discount the 777-200LR or LRF freighters ahead of the 777x isn't really going well, I wonder if we just wait a bit and see if we can get mostly new builds for cheaper?