OK, you claim you fly 75% of the time for work and you found NO other opportunities earlier than December to redeem this credit? Something isn't right there.

I book flights for my staff regularly. If you purchase a Non-refundable ticket, all you can get back is a credit against a future purchase. Since it's NON-REFUNDABLE, they're being generous to do that.

You also don't list who the rep was that told you you had until the end of the year to redeem the credit. Since you have no proof of this, the burden of proof is on you and for all Delta knows, you're making that up to get the credit extended.

I think Red or Tracy are in the travel industry and could clarify that more. But bottom line, they told you there was a deadline, you missed the deadline. You lost out on the credit.