Well, it is pretty much that simple, if you have a contract of employment that said that you would get discounted air travel until death, then sue them! They will soon back down as the legal costs will outweigh the financial gain. If you don't have that, then you have a concession, which is granted by the goodwill of the airline. As such, like my own airline, the management can judge that to remove it saves enough money to outweigh the loss of morale felt by the workforce.
It isn't right or fair, but with food banks springing up all over the country, I am afraid there won't be a lot of public sympathy around for the loss of a non contractual travel perk. (which by the way, is why in my airline the new travel concession has been accepted with barely a whimper of protest)