The DC-9 is a great a/c and continues to perform well with all the airlines that still operate the type. NW is smart to keep them up to date.
I am unable to understand the economics of scrapping a 100+ fleet of older, paid for DC-9s in favor of new 30-50 million jets that require new crew training, new mx training, new facilities, spare parts etc. After operating the DC-9s as long as NW has, I suspect they make good money off utilizing older a/c that were paid for long ago. Eventually of course they will have to replace them with something and I predict (and anybody can comment on this) that Boeing will, out of customer demand develop the 717 into a "family" of a/c like the DC-9/ MD-80. They have done it with every other type they make and there are alot of operators of DC-9s and even more of MD-80s that are very pleased with the economics and customer satisfaction of this type. I can see TWA, NWA, Delta (maybe, but they are already going the way of the crappy 737NG), SAS, SwissAir among others being interested in a larger/smaller 717. If they can possibly justify the 757-300 (which nobody seems to want) then they could justify a further development of the 717.
WHo could ask for more than a type of a/c that performs well for over 30 years and still has a number of years left? I think airlines these days are pretty hasty to replace 10-20 year old a/c, its puzzling.