DC9 -31/2 will be replaced by A318s (NW could go domestic and regionally Airbus), Embraer 190s???, hopefully 717's.
DC9-50/1 will be replaced by A319s, A320s.(Not with Boeing a/c coz of NW's commitment to its present A319,A320 fleet)
These predictions are meant for this decade!! But we'll probably not be surprised to see DC-9's still in operation in 2010
Long Live the DC-9
Photo © Bruce Leibowitz
Photo © TJ OKeefe