Originally Posted by
Herb687
I highly suspect that there's still no way AA could prove, well after the fact, what the available fare was for flight 1234 PHL-DFW at the exact instant that I bought my hypothetical PHL-AUS hidden city itinerary.
....so there's no contest that the passenger paid the "incorrect" fare, right?
Without needing to know what the actual fare would have been, you appear to agree conclusively that the passenger obtained a lower fare.
(Forgive me if your argumentation is that it can't be proved that the fare flown wasn't actually identical to the fare paid in every single such case, and that therefore the passenger paid exactly the right amount...that would appear to be a very weak line of argumentation though...)