The trouble with mixing the fleet like this is many people never look at a schedule and have no idea how AC's planes are configured. Someone who is stuck in the domestic premium Y cabin for 7 hours will think that all AC seats are that close together and will fly Air France next time. A SE who cannot upgrade will defect to another airline. Things like this should matter. If AC advertises a certain product, it should be available on all flights all the time. If they can't offer it, they shouldn't operate the flight. Reputation is a big thing on routes where there is competition.