The reality is that at some point, someone is going to have to make a ~200-220 seat aircraft. It's by far the largest gap in the aircraft market and it's something that will eventually need to be filled. The big question is whether or not DL is going to be able to wait for that to happen. With the recent success rate of Airbus and Boeing launching new aircraft being quite poor, even if something were announced tomorrow (which it likely won't for a while at this point) I would give it about a zero % chance of being available in 2025 when the 767s are on the way out the door. The A321XLR, while fine for tertiary cities/routes like BOS-MAN, JFK-EDI, etc. isn't going to be enough aircraft to replace the 767 on most of the European network.