While my current situation is not as extreme as what you guys are describing, I'm very curious to know the answer to "when is it no longer mechanical?" Where does that line get drawn, when one delay impacts subsequent flights?

My YVR-YUL a couple weeks ago was cancelled, due to a mechanical issue that delayed the scheduled aircraft's earlier (inbound to YVR) flight. Do I have an argument, since the delay occurred several hours before my flight was scheduled, that fleet-utilization is within AC's control and a different aircraft could have operated it instead of cancelling?

AC have of course declined but I am wondering whether it is worth pursuing a CTA claim. (I wish I had the status details from EF but I didn't think to get them in time, unfortunately.)