When does AC drop a flight due to low loads in the absence of bad weather? I've flown this route way too often than I can think of, and been on flights where I'm seriously wondering if AC will be profitable (cue a subbed in 777 for like 30 of us on the plane due to IRROP) or a 319 with like 10 people on it on a nice sunny day.

I believe this claim is unfounded.

Rather than speculate, if you post your flight number in addition to the date you have provided, people here can tell you why it was cancelled. Behind every cancellation a reason is logged, and people on here can easily figure out why.