They wouldn't add the -900 anyway-unless Boeing creates one with more emergency exits, so capacity could be increased in an all coach config.
It is true that the -800 and -900 currently have the same maximum capacity in the most dense all coach arrangement(189 pax) because the -900 does not have enough emergency exits for a higher capacity. But Southwest does not use anything close to the most dense configuration possible(It uses 137 seats rather than 149 seats on the -700), so it would indeed be possible for it to fully utilize the space on the current -900 assuming they use the same seat pitch that they use now.
One other problem with the -900, besides the extra flight attendent, is range. Alaska and other airlines that use it have found that payload can be heavily restricted on long transcons with the -900. Sometimes a fuel stop is required on the "upwind" leg of the journey. This would be a problem for Southwest since the main job of its NG
's is to do longhauls. Most of the shorthauls are still done by -300's.
I believe Boeing is working on a 737-900X to counter these two problems. It will have enough emergency exits to be able to use the bone-jarring seat pitch some charter carriers (not Southwest) use, since it will have extra emergency exits. The structure and engine thrust will be beefed up to handle increased takeoff weight and range. Eventually, this could be of interest to Southwest. But my guess is that they stick with the -700's.