Already said contractual obligation rests with OP, not airline to be prepared to travel with right docs in-hand. Just had this happen, and fortunately Thai in Bangkok helped me at biz counter get an Australian ETA lickety-spit.

More problematic, is the potential consequence of "tossing our useless ticket"?

OP does not explain whether this was one-way booking? or was first outbound leg on a return journey? If yes to either, then OP free to do what they so wish.

OP does provide opportunity to check-out Rule 25(e) in the International Tariff about Prohibited Ticketing practices?

"Throw-away ticketing NOT permitted", thus Air Canada has every right to come back and recalculate the outbound ticket, if outbound was on AC, and ask for payment of the one-way airfare, should this be more than the round-trip airfare originally paid by OP.

Why should frequent travelers care about what OP does? Well, abuse of ticketing rules means AC responds by making even more complex rules to protect their revenues - and we all bear the brunt.

Some might argue well empty seat can be used for standby, and AC gets paid anyway. But AC may be getting paid less than if traveler purchased a more expensive one-way airfare.

Whether you agree with tariff rules or not, this is current contract we have with AC -and I for one would prefer AC avoid responding by making our contracts even more challenging to navigate.

and of course Merry Christmas to OP, and trust this event not cloud your joyous season.