The mandatory retirements in the next 365 days from the left seat of the 747, 777, 767-400 and 330 will accommodate all displaced captains without counting any A350 seats. There could be some pilots at the bottom of the 777 that get displaced if there is a strong Boeing preference, but it should be contained entirely on the 777. Any secondaries of 777 As would be absorbed by the mandatory retirements on the 2x4 and 330. Once the A350 vacancies are factored in, it is a net positive for WB left seaters.

The bigger variable is where the displaced FOs go. I expect many will choose to remain in right seat of a WB, however; there are some who will finish their FO years on the largest airplane flown at Delta and embark on the climb up the left seat ladder, rather than fly a smaller airplane.