Before others chime in to tell you, Delta does not guarantee seat assignments and they state this on Delta.com. Not that I agree with what happened to you, just pointing out what you'll likely be reminded about from other posters.

Having said that, I'd send them a note of complaint as well....

Ask for an explanation calmly, succinctly, and without emotion or threats. Keep to the facts and what information you want from Delta (or what you want them to do, e.g. compensation).

FWIW, I've used such phrases as "disappointed because...", "XYZ...didn't meet the Delta's level of professional standards ", or "what I expected based on Delta's communication to me as late as... " (vis-a-vis boarding pass or confirmed seats).

Don't worry; you'll still get the same auto-fill-in-the-blank response (which you'll likely want to respond to).

This way, once a human actually reads it, your original email will most likely be seen as incredibly reasonable (particularly when compared to the others they get every day). As the saying goes, "...more flies with honey..."

PS: Since there are two of you, remember to include your wife's SM number and her name as it appeared on her ticket. This way one email = compensating you both (hopefully).