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They are streamlining their long haul fleet to make most economic sense, not to a particular manufacturer. Airlines today are ordering a greater variety of planes that suit their needs more specifically, whereas in the past they would order 1 or 2 types and use them with slightly less efficiency.
Take a look at the British Airways fleet. 5 years ago they operated the 767-300, 777-200 and 747-400. In 5 years they will have a fleet of 787-8, 787-9, 787-10, 777-200, 777-300ER, 747-400, A350-1000 and A380-800. Eventually the 747-400 and 777-200 will leave, but it confirms that they are looking for planes that suit particular routes a lot more than 10 years ago.
American Airlines will go from a fleet of 777-200 and 767-300 to a fleet of 767-300, 777-200, 777-300ER, 787-8, 787-9, A330-200, A330-300 and A350-900.
I am not certain that Delta will keep it's order of 787-8s either, but they need to replace 58 767-300ER, and the 787-8 is a much closer match than the A330-900, which is only slightly smaller than a 777. Some routes can sustain a bigger plane, but some certainly can't, and the 757-200 routes probably won't. Furthermore, they ordered the 787s at a time when 787s were far cheaper than today, so they are actually a really great bargain.