Welcome to FlyerTalk, BostonAAflyer

Can I ask that you expand a few points - this will allow us to help you.

It is clear you were booked on a IB operated flight from BOS to MAD on 31 March, you had checked in and were waiting to board. Can I ask where you were waiting? From your post it does read you may have been in a secluded place relying on AA for flight updates to be delivered to your phone/laptop/iDevice. During this time the departure boards must have shown the cancellation otherwise the other passengers who had queued to rebook would not have known to approach IB to rebook.

Your assumption that any airline will cancel a flight at the last minute because of low passenger numbers really is a false one. When an airline cancels a flight, the aircraft, the flight crew and cabin crew are then placed out of position. This in itself can create enormous operational problems - the aircraft and crews operating the IB6166 on 31 March would have been needed to be in MAD the next morning just as much as their passengers.