DL doesn't compete against NK. It's that simple.
Those fare comparisons are pretty much true on pretty much every route NK flies, whether it's up against UA, AA, or DL.
E fares are E fares. People who need to save, use them. At least they get a safe aircraft and IRROPS rebooking. Others stick with the standard fares and DL does quite well with them.
If your marketing data shows otherwise, maybe DL will drop its fares. But, DL does pretty well at pricing. So, my guess is that DL is right within a few bucks and DL will do just fine.