In any follow-up complaint it is worth asking why wasn’t anyone upgraded to the 3 empty FC seats.
This is to avoid triggering AI into referring to the terms that influence your priority.

I would also propose a solution in the complaint: if there are no shows to the assigned seats, the crew should have a short list of passengers eligible for upgrade to do it manually after the boarding is closed.

Of course, question from 1 or 2 customers will generate general replies and maybe some consolation miles. If more of us point at the same problem and solution the chances are higher that it becomes a business case for Delta to implement the process improvement.