Play their bluff & remember last on first off
A very scary attitude which may end with an airline closing. The last on first off rule means nothing in this day and age of FWA. A company can easily (when losing hundreds of millions of dollars), seek exemption from that rule, or any other actually, and will probably get it. If the unions are telling you different they are not being straight with you.
Never rule out negotiating. Otherwise your bluff just might backfire and it is you who will be bluffed.