The 739ER order is official now and note that DL says it will be for replacing 757s, 767s, 320s, but not M80s.
Much of DL's leased fleet has lease expirations coming up in 2013-2014 which is 6-7 years after DL left bankruptcy. Presumably, the combination of the age of the aircraft that will be replaced coincide. Since the M80s/D9s are structurally capable of flying for 30 years or more, perhaps DL intends to renew leases on the M80s likely at even lower rates than what DL pays now. Because the M80s typically fly east coast business market routes with lower rates of connections, it is quite likely that DL is able to make money with that fleet type even though it is one of the least fuel efficient... or perhaps the Cseries or Ejet order will be made with the intention of replacing the M80s. Since Boeing Bombardier and Embraer are likely capable of supplying whatever amount of aircraft DL might want, there isn't a need to hurry... plus the product offerings continue to evolve.
"As a result of maintenance efficiencies and a 15 to 20 percent improvement in fuel consumption per seat, the Boeing 737-900ER will have lower unit costs than the older technology Boeing 757 and 767 and Airbus A320 aircraft that it will replace."
It should be noted that DL's 739ER deliveries come just as its $2B international fleet renewal project winds down, after the ATL, LGA, and JFK terminal projects (current phases should be completed), and that at a max of 20 aircraft per year, DL's capex outlays should be about $1B per year, similar to levels it has been spending which has allowed it to reduce its debt levels.
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DL's 739ER purchase seems just enough to replace what has to be replaced with the lowest cost alternative available. DL says the aircraft replacements will be capacity neutral and would still leave DL in a position to order an all new narrowbody should Boeing offer it 5-10 years from now.
