If DL has refiled higher fares in the same buckets, you might have an argument (but not a legal case). There is a highly publicized supply-demand mismatch happening currently among US airlines. It seems far more likely to me that this is reflective of availability issues and selling out lower fare buckets sooner than in previous years. "People are still dying" is unlikely to get you very far with res or rev management, unless you require a bereavement fare in which case you would be entitled to the standard 10% discount.

I am seeing plenty of K fares between $550 and $650, even on your dates. Leaving a day earlier looks to result in about $100 savings.