I would have liked to be מעיין in this, but there is just not enough time in the day. Oh well...
Just some comments/questions:
1. I see people discussing אונאה, I don't know how this is אונאה, it is a טעות.
אונאה is when the seller (or buyer) is not aware of the true market value, here they were aware, and just made a mistake.
This point changes the dynamics of the whole issue.
השבת אבידה is much more applicable here, as in the OP. The issue of הצלה מהפסד is an offshoot of השבת אבדה, not אונאה.
[2. I wonder if אונאה would even apply to a service provided, as opposed to sale (or rental) of goods. An airline flight is technically a service, not the purchase of merchandise.
3. אונאה can be נמחל. So, if the whole issue here was only אונאה, there would be no problem to keep the ticket after El-Al announced that they are honoring them.]