Nothing. AC's T&C regarding last minute upgrades are pretty crystal clear:

• Prices are based on each individual flight segment for which a Last Minute Upgrade Purchase is requested.
• Last Minute Upgrades are not available on all flights and markets and subject to availability in the Executive cabin.
• Prices vary by flight and by market and will only be determined and presented to the customer during the check-in process. The customer will have final purchase decision for all eligible flight segments of his itinerary.
• The Last Minute Upgrade Purchase fee is subject to all applicable taxes.
• Passengers accumulate mileage, including Air Canada status mileage, as per original fare option they purchased.
• Air Canada cannot guarantee to provide Business Class meal service to customers who make a Last Minute Upgrade Purchase.

You have every right to be annoyed. But I don't think AC is obligated to compensate you.