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Delta did not cancel the 787 order (yet). They ordered 25 A350-900 and 25 A330-900, and it is rather the A330-900s that challenged the 787. The A330-900 will replace the oldest 767-300 in the Delta fleet. The A350 is a class above the 787, and will replace the older 777s, the 747s and the 767-400s.
Delta has 95 767s of all variants that need replacement, as well as 8 777-200ER and 13 747-400 and 18 757-200 that are used on medium haul routes. 16 767-300As will be replaced by A321s and 737-900ERs. That leaves 118 planes to replace. They will receive the 50 planes from this order, plus 10 A330-300 from a previous order, leaving another 58 to replace. They will need the 787-8s or more A330NEOs, and I believe the 787-8s are already paid for. The 787-8 is also smaller than the A330-900, making it more suitable to replace the 757-200s.