I doubt NW will leave MDW. DL just started flying MDW-ATL about 6-9 months ago with EMB170's with both F and Y. NW has a much bigger operation at MDW, with mainline jets. And now with the DL combination - the new DL will have lots of flights to/from MDW.

CO used to be all mainline jets to/from MDW, before the MDW terminal was rebuilt, but with the shift to RJ's - CO's market share has dropped. Fare pressure from WN and CO with limited RJ seats - CO can not compete there.

I liked flying into MDW, but I usually departed out of ORD because of the PC. I have flown NW from DTW/MDW many times, and upgrades are easy on that route.