A359 is similar size as 781, which AC has ordered. In theory A351 could be 77w replacement, oldest being 17 years old, so AC might use them for another 10 years.
Ding Ding.
This question has come up frequently in some of the groups I'm in and the question I often pose to the poster is, "aside from I want AC to have the newest toy, how would an A350 sub fleet fit into AC's existing (and future) fleet in a more economically profitable way than say 787's and 77X's?"
It just seems so obvious that the 787 fleet will be the backbone of AC's widebody long haul fleet for the next couple of decades, their ultra long haul fleet (772L/773E) have a good amount of life in them, and even when it comes time to replace them the most obvious replacement would be the 777X (barring a new more suitable aircraft coming out between now and them)