There were two factors that helped swung the decision in favour of the A350.
1. There’s a greater distance between doors 1 & 2 on the A350-1000 which enables a better premium configuration for the ULR routes, &
2. The basic weight of the A350-1000 is 10 tonnes lighter thus saving fuel over the life of the aircraft. Being so much lighter, I don’t expect them to be strong enough to remain in service as long as the B777-8 would.
With the post COVID business being completely different, the business case for the aircraft would have to be rerun if they were looking for additional capacity.
The remaining B787 options are approximately half the cost of the A350 so they would have to be considered for non ULR routes.