I'm not exactly sure why AC is surprised using the benefit unfairly impacts other customers? If Y > 0, then the benefit would go unused, and no unfair impact occurs. If its Y=0 at T-6, then there's a reason to use the benefit, which probably lead to an oversell situation at the gate. If people are using the benefit in advance of T-6, then I'd expect changes on the day of the flight (or in advance) to balance things out and there's less of an oversell situation. But in those cases, people with the benefit might as well just wait to see if inventory becomes available, and if it doesn't just call in to use the benefit at T-6.
I guess it's a benefit AC wanted to offer without having people actually use it. It would have been nice if they actually provided some other "enhancement" than just removing a benefit (part way through the benefit year).
Discontinuing the Guaranteed Reservations benefit
The Guaranteed Reservations benefit for Aeroplan 75K and Aeroplan Super Elite Members in Economy Class on Air Canada flights is no longer offered. This benefit allowed members to obtain a guaranteed seat when they completed a new booking on an eligible flight with the purchase of a Latitude fare.
The benefit was first introduced over a decade ago when airlines commonly overbooked their flights. This is no longer the case.
While this benefit is rarely used by members, we found that these bookings unfairly impacted other customers. By removing it, we are better able to align our service offering for all our customers.
Removing it does not affect our ability to accommodate you on a new flight when a flight disruption occurs.