The A321 can be stretched to cover the MOM segment, with the engine that Delta is now the MRO for. Neither a reissued 757 or 737 can meet takeoff certification requirements, without sacrificing fuel economy in this market, thus requiring a entirely new designed 797. I don't know if it makes financial sense for Delta to buy the 797, when these 100 options can be converted to an A321neo stretch with the volume discount. Obviously, more than just 75 Cseries are coming as well, so it's going to be all Airbii for a while.

Also, before the merger, I was working on the 787 Goldcare MRO at NWA, which Boeing screwed Delta out of. I don't think we will ever see the 787 in Delta colors for that reason.