Yes, eventually the DC 9's will be replaced. I know that prior to the merger NWA was heavily evaluating the C series. I doubt you will see any aircraft purchases until boeing and airbus announce their competing products. In the short term DAL is better off using the DC9 to put the squeeze on the competition where they can, maybe they are planning on another airline going TU. However, in the past that's been a losing strategy. Another thing to think about would be, if UA or US goes bankrupt, could another 1113c get 100 seat scope relief from the pilots? If that happens, I doubt you will see the DC9 replacement at mainline.