In Spanners defence, CX had been consistently saying that European expansion would come with the A350. The pointers suggested for around 100,000 passengers per year so it would have been logical to have a smaller aircraft to start the route. I doubt they would have anticipated getting 300 passengers per flight which is above the A350 capacity.
Unfortunately, DUS getting the A350 does nothing for freeing up a 77W for MAN - its the same days of operation but with the 4 class 77W.