JFK-DUB utilizes both the 752 and 763ER deepening on the season.

To me the answer is simple. Delta wants capacity discipline. The 787 provides the seating of a 763 and range to take those passengers most anywhere that DL serves. It seems like the obvious choice. A few 77W that can be ordered on the cheap make sense for high density hub-hub flights like ATL/JFK-NRT. Otherwise use the smaller 787's and you can increase the ticket price due to limited seating. I'm baffled why DL ordered more A333's. It's a HUGE airplane. DL has been chanting they are all about capacity discipline. It doesn't make sense. The fewer seats on aircraft the higher the price you can command. The 787 has the seating capacity of a 767 but can take those passengers much further. It just makes sense for the 787 to make up the bulk of a long-haul fleet.