Advantages of the A350-1000 vs 77X:

• Longer range
• Comparable operating economics with a lower total seat count
• Might be hard filling the 77X on many routes - few of AC's routes are slot constrained
• Greater passenger comfort (seat and/or aisle width)
• 3-3-3 configuration versus 3-4-3 means 1/3 of seats are less desirable middle seats versus 40%
• I think the only area where the 77X is likely to win out is in freight hauling capacity

At the end of day it will likely come down to who offers the best deal, but at current fuel prices there is no mad rush to get rid of the 77W.