Update on Delta's response. I interacted with online chat and this is the summary:

The agent said his research showed the initial credit should have had an earlier expiration date than the 2025 date on the credit and that it should have been even sooner than the expiration date I was challenging. He did spend a lot of time away from the chat, so I suspect he did research the history of the credit. That could very well be true as this was a credit that I had not followed as well as others.

I feared that to pursue further might have resulted in my loss of the credit so I went ahead and applied it to a new ticket. Thank goodness I have the flexibility to do so. I wasn't up for an extended back and forth that I might have lost.

Not an optimal solution and I think DL should be held to the expiration date they send with the eCredit.