I don't see how under US law a regular ticket you purchase can ever become worthless. Courts have ruled that there can be no expiration for gift certificates, for instance. I would imagine you can get the airline to re-issue.

I had some old upgrade coupons on AA and they cheerfully accepted them without a second thought. After all, I had paid for them one time or another. Why should the money I paid all of a sudden become worthless (that's what inflation is for :-))