This is the perfect example of "them the rules" and, personally, I find it incredibly angering. If I ran my business based on rules alone, I'd quickly find myself on the street.
These kinds of "rules," especially those pertaining to customer loyalty, should be guidelines as opposed to set in stone. How often do we hear of flyers getting comped status despite having missed the threshold by a few hundred (or more) miles? Meanwhile, OP has a demonstrated loyalty to AC and gets a slap in the face.
I sincerely wish someone at AC had the courage to do the right thing for the OP.