Dandy_Don, I got to hand it to you, at least you asked this topic in a good way. You actually want to make it intelligent, instead of the usual. "When's NW gonna get those 717's" This topic is discussed weekly, but at least you are asking a valid question on whether it COULD work.
The thing about the DC-9 fleet is that it is so varied. NW operates the 10, 30, 40, and 50 versions.
Current DC-9 Fleet: 169
-10's 8 a/c 78 pax
-30's 114 a/c 100 pax
-40's 12 a/c 110 pax
-50's 35 a/c 125 pax
The replacement suitable for the -50's would be the A319, which holds 124 pax. However, the -50's are significantly newer, and less cycles than the older 10's, 30's, and 40's. The -50's are from the late 70's & early 80's. As stated before in early posts about the DC-9 retirement schedule has no set dates or anything like that. Around 100,000 cycles they will be pulled from service. Also, 75 A319 replacements.
I take warning when saying this, but yes, the 717 COULD work for SOME of the DC-9 replacement. I'm not saying it will or anything like that or it fits with their fleet or whatnot.....but from a purely factual basis, yes. Its range could cover the low end DC-9 routes and capacity-wise at the smaller end of the spectrum. With the addition of the A319's, the stage length of the DC-9's routes have been shortened. The A319's have the ability to fly both long and short. For the most part, they are flying the longer routes. One exception would be DTW-GRR (120 miles)..the DC-9's are left for the short hops such as DTW-LAN, DTW-FNT, DTW-CLE, however there are some -9's going to Florida from DTW and to the East Coast from MSP. This just goes to show that the DC-9's are all over the place.
As its been said before, the fellas up in MSP know what they are doing. Since they will have 75 A319's to replace the first of the DC-9's to be retired, they puts them in pretty good shape for a while. They will worry about the remainder when the time comes, and see what the market holds.