I've read that the Morris Air acquisition, while seemingly successful on the surface, was a real challenge behind the scenes, and had people in Dallas promising to never do it again.

Southwest is now growing at a 10% clip; depending on Boeing's production ability, they can probably keep up this growth rate, and perhaps grab a few used 737s as other carriers merge.

While the ATA move was apparently successful, and trying to acquire other carriers is a tempting proposition, steady-as-she-goes is probably the right course for them.